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Pair of Linear Equations in Two Variables — Important Questions

13 hand-picked CBSE Class 10 Maths important questions for Pair of Linear Equations in Two Variables, each with a full model answer — the formats and topics most likely to appear in your board exam.

13
Questions
6
Question types
32
Total marks
₹0
With answers
Quick answer

Focus on the consistency conditions comparing a1a2, b1b2, c1c2\frac{a_1}{a_2},\ \frac{b_1}{b_2},\ \frac{c_1}{c_2} (unique / no / infinite solutions), finding kk for parallel or coincident lines, graphical solutions with triangle areas, and word problems on fractions, ages, speed–stream and cost solved by substitution or elimination.

About Pair of Linear Equations in Two Variables

A pair a1x+b1y+c1=0, a2x+b2y+c2=0a_1x+b_1y+c_1=0,\ a_2x+b_2y+c_2=0 is pictured as two straight lines. Comparing the ratios of the coefficients tells you at a glance whether they intersect once, coincide, or are parallel, while substitution and elimination convert real-life situations into solvable systems.

Graphical method and consistencySubstitution and eliminationConditions on coefficient ratiosEquations reducible to linear formWord problems (age, speed, fraction, cost)

Key concepts & formulas

Three cases from the ratios

For a1x+b1y+c1=0a_1x+b_1y+c_1=0 and a2x+b2y+c2=0a_2x+b_2y+c_2=0: a unique solution (intersecting lines) if a1a2b1b2\dfrac{a_1}{a_2}\neq\dfrac{b_1}{b_2}; infinitely many (coincident) if a1a2=b1b2=c1c2\dfrac{a_1}{a_2}=\dfrac{b_1}{b_2}=\dfrac{c_1}{c_2}; no solution (parallel) if a1a2=b1b2c1c2\dfrac{a_1}{a_2}=\dfrac{b_1}{b_2}\neq\dfrac{c_1}{c_2}.

Elimination method

Make the coefficients of one variable equal by multiplying, then add or subtract to eliminate that variable. It is usually the fastest route for board word problems.

Reducible equations

Equations such as ax+by=c\dfrac{a}{x}+\dfrac{b}{y}=c become linear on substituting u=1x, v=1yu=\dfrac{1}{x},\ v=\dfrac{1}{y}; solve for u,vu,v and then back-substitute to get x,yx,y.

Setting up word problems

Assign a variable to each unknown and turn every sentence into one equation. Common templates: speed =distancetime=\dfrac{\text{distance}}{\text{time}} (boat–stream: upstream uvu-v, downstream u+vu+v); a two-digit number =10t+u=10t+u.

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Important questions with answers

Try each on paper first, then reveal the model answer to check your method.

Question typeCountMarks
MCQ41
Assertion–Reason11
Very Short22
Short Answer33
Long Answer25
Case-based14

Multiple-choice questions (1 mark)

Q1MCQEasy1 mark

The pair of equations x+2y=5x+2y=5 and 3x+6y=153x+6y=15 has:

  1. (a)

    a unique solution

  2. (b)

    exactly two solutions

  3. (c)

    infinitely many solutions

  4. (d)

    no solution

Show model answer

Answer: (c) infinitely many solutionsa1a2=13, b1b2=26=13, c1c2=515=13\dfrac{a_1}{a_2}=\dfrac{1}{3},\ \dfrac{b_1}{b_2}=\dfrac{2}{6}=\dfrac{1}{3},\ \dfrac{c_1}{c_2}=\dfrac{5}{15}=\dfrac{1}{3}. All three ratios are equal, so the lines are coincident.

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Q2MCQEasy1 mark

The graph below shows two lines representing a pair of linear equations. The pair is:

CBSE Class 10 Maths — Pair of Linear Equations in Two Variables: The graph below shows two lines representing a pair of linear equations. The pair is:
  1. (a)

    consistent with a unique solution

  2. (b)

    inconsistent

  3. (c)

    consistent with infinitely many solutions

  4. (d)

    dependent

Show model answer

Answer: (a) consistent with a unique solution — The two lines intersect at exactly one point (2,3)(2,3), so the system is consistent and has a unique solution.

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Q3MCQModerate1 mark

The value of kk for which the pair 2x+3y=72x+3y=7 and (k1)x+(k+2)y=3k(k-1)x+(k+2)y=3k has infinitely many solutions is:

  1. (a)

    k=3k=3

  2. (b)

    k=5k=5

  3. (c)

    k=7k=7

  4. (d)

    k=7k=-7

Show model answer

Answer: (c) k=7k=7 — For infinitely many solutions 2k1=3k+2=73k\dfrac{2}{k-1}=\dfrac{3}{k+2}=\dfrac{7}{3k}. From 2k1=3k+2\dfrac{2}{k-1}=\dfrac{3}{k+2}:  2(k+2)=3(k1)k=7\ 2(k+2)=3(k-1)\Rightarrow k=7. This also satisfies 73k=721=13=26\dfrac{7}{3k}=\dfrac{7}{21}=\dfrac{1}{3}=\dfrac{2}{6}. ✓

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Q4MCQHOTS1 mark

The pair of equations 3xy=53x-y=5 and 6x2y=k6x-2y=k has no solution for:

  1. (a)

    k=10k=10

  2. (b)

    all k10k\neq10

  3. (c)

    k=10k=-10

  4. (d)

    all real kk

Show model answer

Answer: (b) all k10k\neq10 — Here a1a2=36=12\dfrac{a_1}{a_2}=\dfrac{3}{6}=\dfrac{1}{2} and b1b2=12=12\dfrac{b_1}{b_2}=\dfrac{-1}{-2}=\dfrac{1}{2} are equal. For no solution we need c1c212\dfrac{c_1}{c_2}\neq\dfrac{1}{2}, i.e. 5k12k10\dfrac{5}{k}\neq\dfrac{1}{2}\Rightarrow k\neq10.

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Assertion–Reason questions (1 mark)

Q5Assertion–ReasonModerate1 mark

Assertion (A): The pair of equations x+2y4=0x+2y-4=0 and 2x+4y12=02x+4y-12=0 has no solution.

Reason (R): A pair of linear equations has no solution when a1a2=b1b2c1c2\dfrac{a_1}{a_2}=\dfrac{b_1}{b_2}\neq\dfrac{c_1}{c_2}.

  1. (a)

    Both A and R are true and R is the correct explanation of A

  2. (b)

    Both A and R are true but R is not the correct explanation of A

  3. (c)

    A is true but R is false

  4. (d)

    A is false but R is true

Show model answer

Answer: (a) Both are true and R explains A. The ratios are 12=24=12\dfrac{1}{2}=\dfrac{2}{4}=\dfrac{1}{2} but c1c2=412=1312\dfrac{c_1}{c_2}=\dfrac{-4}{-12}=\dfrac{1}{3}\neq\dfrac{1}{2}, so the lines are parallel and the system has no solution.

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Very short answer questions (2 marks)

Q6Very ShortEasy2 marks

For what value of kk will the lines 3x+2ky=23x+2ky=2 and 2x+5y+1=02x+5y+1=0 be parallel?

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For parallel lines a1a2=b1b2\dfrac{a_1}{a_2}=\dfrac{b_1}{b_2} (with the constant ratio unequal). Thus 32=2k54k=15k=154\dfrac{3}{2}=\dfrac{2k}{5}\Rightarrow 4k=15\Rightarrow k=\dfrac{15}{4}.

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Q7Very ShortEasy2 marks

Solve the pair of equations 2x+y=232x+y=23 and 4xy=194x-y=19. Hence find the value of 5y2x5y-2x.

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Adding the two equations: 6x=42x=76x=42\Rightarrow x=7. Then y=232(7)=9y=23-2(7)=9.

So 5y2x=5(9)2(7)=4514=315y-2x=5(9)-2(7)=45-14=31.

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Short answer questions (3 marks)

Q8Short AnswerModerate3 marks

A fraction becomes 13\dfrac{1}{3} when 11 is subtracted from its numerator, and it becomes 14\dfrac{1}{4} when 88 is added to its denominator. Find the fraction. (CBSE 2023)

Show model answer

Let the fraction be xy\dfrac{x}{y}.

x1y=133xy=3...(i)\dfrac{x-1}{y}=\dfrac{1}{3}\Rightarrow 3x-y=3\quad\text{...(i)}

xy+8=144xy=8...(ii)\dfrac{x}{y+8}=\dfrac{1}{4}\Rightarrow 4x-y=8\quad\text{...(ii)}

Subtracting (i) from (ii): x=5x=5. Then y=3x3=12y=3x-3=12.

The fraction is 512\dfrac{5}{12}.

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Q9Short AnswerModerate3 marks

Solve for xx and yy:  2x+3y=13\ \dfrac{2}{x}+\dfrac{3}{y}=13 and 5x4y=2\dfrac{5}{x}-\dfrac{4}{y}=-2.

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Let u=1xu=\dfrac{1}{x} and v=1yv=\dfrac{1}{y}:  2u+3v=13 ...(i)\ 2u+3v=13\ \text{...(i)},  5u4v=2 ...(ii)\ 5u-4v=-2\ \text{...(ii)}.

Multiply (i) by 44 and (ii) by 33:  8u+12v=52\ 8u+12v=52 and 15u12v=615u-12v=-6. Adding: 23u=46u=223u=46\Rightarrow u=2.

From (i): 3v=134=9v=33v=13-4=9\Rightarrow v=3.

So x=1u=12x=\dfrac{1}{u}=\dfrac{1}{2} and y=1v=13y=\dfrac{1}{v}=\dfrac{1}{3}.

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Q10Short AnswerHOTS3 marks

A boat goes 3030 km upstream and 4444 km downstream in 1010 hours. In 1313 hours it can go 4040 km upstream and 5555 km downstream. Find the speed of the boat in still water and the speed of the stream.

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Let the boat's speed =u=u km/h and the stream's speed =v=v km/h. Put p=1uvp=\dfrac{1}{u-v} and q=1u+vq=\dfrac{1}{u+v}.

30p+44q=10 ...(i)30p+44q=10\ \text{...(i)},  40p+55q=13 ...(ii)\ 40p+55q=13\ \text{...(ii)}.

Multiply (i) by 44 and (ii) by 33: 120p+176q=40120p+176q=40 and 120p+165q=39120p+165q=39. Subtracting: 11q=1q=111u+v=1111q=1\Rightarrow q=\dfrac{1}{11}\Rightarrow u+v=11.

From (i): 30p+4=10p=15uv=530p+4=10\Rightarrow p=\dfrac{1}{5}\Rightarrow u-v=5.

Solving: u=8, v=3u=8,\ v=3. Boat =8=8 km/h, stream =3=3 km/h.

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Long answer questions (5 marks)

Q11Long AnswerModerate5 marks

Solve graphically the pair of equations 2x+y=62x+y=6 and 2xy+2=02x-y+2=0. Shade the triangle formed by these two lines and the xx-axis, and find its area.

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For 2x+y=62x+y=6: points (0,6)(0,6) and (3,0)(3,0). For 2xy+2=02x-y+2=0, i.e. y=2x+2y=2x+2: points (0,2)(0,2) and (1,0)(-1,0).

The lines meet where 2x+y=62x+y=6 and 2xy=22x-y=-2; adding, 4x=4x=1, y=44x=4\Rightarrow x=1,\ y=4. So they intersect at (1,4)(1,4).

The triangle formed with the xx-axis has vertices (1,0), (3,0)(-1,0),\ (3,0) and (1,4)(1,4), with base =3(1)=4=3-(-1)=4 units and height =4=4 units.

Area=12×4×4=8 sq units.\text{Area}=\tfrac12\times4\times4=8\ \text{sq units.}

CBSE Class 10 Maths — Pair of Linear Equations in Two Variables: Solve graphically the pair of equations 2x+y=6 and 2x-y+2=0. Shade the triangle formed by these two lines and the x
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Q12Long AnswerHOTS5 marks

Places AA and BB are 100100 km apart on a highway. One car starts from AA and another from BB at the same time. If the cars travel in the same direction at different speeds, they meet in 55 hours; if they travel towards each other, they meet in 11 hour. Find the speeds of the two cars.

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Let the speeds be uu km/h and vv km/h with u>vu>v.

Same direction (relative speed uvu-v): 100100 km covered in 55 h 5(uv)=100uv=20 ...(i)\Rightarrow 5(u-v)=100\Rightarrow u-v=20\ \text{...(i)}.

Opposite directions (relative speed u+vu+v): 1(u+v)=100u+v=100 ...(ii)1(u+v)=100\Rightarrow u+v=100\ \text{...(ii)}.

Adding (i) and (ii): 2u=120u=602u=120\Rightarrow u=60; then v=40v=40.

The speeds are 6060 km/h and 4040 km/h.

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Case-based questions (4 marks)

Q13Case-basedModerate4 marks

A taxi service charges a fixed amount plus a rate per kilometre. Riya paid ₹105105 for a 1010 km ride and ₹155155 for a 1515 km ride.

(i) Taking the fixed charge as ₹xx and the per-kilometre rate as ₹yy, form the pair of linear equations.
(ii) Find the fixed charge and the rate per kilometre.
(iii) How much will a 2525 km ride cost?

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(i) x+10y=105 ...(i)x+10y=105\ \text{...(i)} and x+15y=155 ...(ii)x+15y=155\ \text{...(ii)}.

(ii) Subtracting (i) from (ii): 5y=50y=105y=50\Rightarrow y=10. Then x=10510(10)=5x=105-10(10)=5. So the fixed charge is ₹55 and the rate is ₹1010 per km.

(iii) Cost of a 2525 km ride =x+25y=5+25(10)==x+25y=5+25(10)=255255.

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