Finding the Unknown | FIO

Question 4

Find the value of each unknown:

(a) 2y=602y = 60

(b) 8=5x3-8 = 5x - 3

(c) 53w=15-53w = -15

(d) 13z=813 - z = 8

(e) k+8=12kk + 8 = 12 - k

(f) 7m=m37m = m - 3

(g) 3n=10+n3n = 10 + n

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Solution
Understand the Question
  • To solve a linear equation for an unknown variable, isolate the variable on one side by applying inverse operations (addition/subtraction, multiplication/division) equally to both sides.
  • When variables appear on both sides of the equation, collect all variable terms on one side and constant terms on the other before solving.

(a) 2y=602y = 60

Step 1 · Solve for yy

Given equation 2y=602y = 60

Divide both sides by 22

2y2=602y=30\begin{aligned} \dfrac{2y}{2} &= \dfrac{60}{2} \\[0.6em] y &= 30 \end{aligned}
Answer

(a) y=30y = 30

(b) 8=5x3-8 = 5x - 3

Step 1 · Solve for xx

Given equation 8=5x3-8 = 5x - 3

Add 33 to both sides

8+3=5x3+35=5x\begin{aligned} -8 + 3 &= 5x - 3 + 3 \\ -5 &= 5x \end{aligned}

Divide both sides by 55

55=5x5x=1\begin{aligned} \dfrac{-5}{5} &= \dfrac{5x}{5} \\[0.6em] x &= -1 \end{aligned}
Answer

(b) x=1x = -1

(c) 53w=15-53w = -15

Step 1 · Solve for ww

Given equation 53w=15-53w = -15

Divide both sides by 53-53

53w53=1553w=1553\begin{aligned} \dfrac{-53w}{-53} &= \dfrac{-15}{-53} \\[0.6em] w &= \dfrac{15}{53} \end{aligned}
Answer

(c) w=1553w = \dfrac{15}{53}

(d) 13z=813 - z = 8

Step 1 · Solve for zz

Given equation 13z=813 - z = 8

Subtract 1313 from both sides

13z13=813z=5\begin{aligned} 13 - z - 13 &= 8 - 13 \\ -z &= -5 \end{aligned}

Divide both sides by 1-1

z1=51z=5\begin{aligned} \dfrac{-z}{-1} &= \dfrac{-5}{-1} \\[0.6em] z &= 5 \end{aligned}
Answer

(d) z=5z = 5

(e) k+8=12kk + 8 = 12 - k

Step 1 · Solve for kk

Given equation k+8=12kk + 8 = 12 - k

Add kk to both sides

k+8+k=12k+k2k+8=12\begin{aligned} k + 8 + k &= 12 - k + k \\ 2k + 8 &= 12 \end{aligned}

Subtract 88 from both sides

2k+88=1282k=4\begin{aligned} 2k + 8 - 8 &= 12 - 8 \\ 2k &= 4 \end{aligned}

Divide both sides by 22

2k2=42k=2\begin{aligned} \dfrac{2k}{2} &= \dfrac{4}{2} \\[0.6em] k &= 2 \end{aligned}
Answer

(e) k=2k = 2

(f) 7m=m37m = m - 3

Step 1 · Solve for mm

Given equation 7m=m37m = m - 3

Subtract mm from both sides

7mm=m3m6m=3\begin{aligned} 7m - m &= m - 3 - m \\ 6m &= -3 \end{aligned}

Divide both sides by 66

6m6=36m=12\begin{aligned} \dfrac{6m}{6} &= \dfrac{-3}{6} \\[0.6em] m &= -\dfrac{1}{2} \end{aligned}
Answer

(f) m=12m = -\dfrac{1}{2}

(g) 3n=10+n3n = 10 + n

Step 1 · Solve for nn

Given equation 3n=10+n3n = 10 + n

Subtract nn from both sides

3nn=10+nn2n=10\begin{aligned} 3n - n &= 10 + n - n \\ 2n &= 10 \end{aligned}

Divide both sides by 22

2n2=102n=5\begin{aligned} \dfrac{2n}{2} &= \dfrac{10}{2} \\[0.6em] n &= 5 \end{aligned}
Answer

(g) n=5n = 5

Common Mistakes
  • Sign Errors with Negatives: Forgetting that dividing a negative number by another negative number yields a positive result (e.g. 1553=1553\dfrac{-15}{-53} = \dfrac{15}{53} and 51=5\dfrac{-5}{-1} = 5).
  • Transposing Variables: Forgetting to change the sign of terms when moving them across the equals sign (e.g. in k+8=12kk + 8 = 12 - k, adding kk to both sides gives 2k2k, not 00).

More questions in FIO

Q1

Solve these equations and check the solutions.

(a) 3x10=353x - 10 = 35 (b) 5s=3s5s = 3s (c) 3u7=2u+33u - 7 = 2u + 3 (d) 4(m+6)8=2m44(m + 6) - 8 = 2m - 4 (e) u15=6\dfrac{u}{15} = 6

Q2

Frame an equation that has no solution.

[Hint: 4 more than a number, and 5 more than a number can never be equal!]

Q3

Write 5 equations whose solution is x=2x = -2.

Q4

Find the value of each unknown:

(a) 2y=602y = 60

(b) 8=5x3-8 = 5x - 3

(c) 53w=15-53w = -15

(d) 13z=813 - z = 8

(e) k+8=12kk + 8 = 12 - k

(f) 7m=m37m = m - 3

(g) 3n=10+n3n = 10 + n

Q5

I am a 3-digit number. My hundred's digit is 3 less than my ten's digit. My ten's digit is 3 less than my unit's digit. The sum of all the three digits is 15. Who am I?

Q6

The weight of a brick is 1 kg more than half its weight. What is the weight of the brick?

Q7

One quarter of a number increased by 9 gives the same number. What is the number?

Q8

Given 4k+1=134k + 1 = 13, find the values of:

(a) 8k+28k + 2

(b) 4k4k

(c) kk

(d) 4k14k - 1

(e) k2-k - 2

Q9

Fill in the blanks with integers.

(a) 5×8=375 \times \underline{\quad} - 8 = 37

(b) 37(33)=3537 - (33 - \underline{\quad}) = 35

(c) 3×(11+)=45-3 \times (-11 + \underline{\quad}) = 45

Q10

Ranju is a daily wage labourer. She earns ₹ 750 a day. Her employer pays her in 50 and 100 rupee notes. If Ranju gets an equal number of 50 and 100 rupee notes, how many notes of each does she have?

Q11

In the given picture, each black blob hides an equal number of blue dots. If there are 25 dots in total, how many dots are covered by one blob? Write an equation to describe this problem.

Q12

Here are machines that take an input, perform an operation on it and send out the result as an output.

Q13

What are the inputs to these machines?

Q14

A taxi driver charges a fixed fee of ₹800 per day plus ₹20 for each kilometer traveled. If the total cost for a taxi ride is ₹2200, determine the number of kilometres traveled.

Q15

The sum of two numbers is 76. One number is three times the other number. What are the numbers?

Q16

The figure shows the diagram for a window with a grill. What is the gap between two rods in the grill?

Q17

In a restaurant, a fruit juice costs ₹15 less than a chocolate milkshake. If 4 fruit juices and 7 chocolate milkshakes cost ₹600, find the cost of the fruit juice and milkshake.

Q18

Given 28p36=9828p - 36 = 98, find the value of 14p1914p - 19 and 28p3828p - 38.

Q19

The steps to solve three equations are shown below. Identify and correct any mistakes.

(a) 6x+9=666x + 9 = 66

x+9=11x + 9 = 11

x=119x = 11 - 9

x=2x = 2

(b) 14y+24=3614y + 24 = 36

7y+12=187y + 12 = 18

7y=67y = 6

y=67y = \dfrac{6}{7}

(c) 4x5=9x+84x - 5 = 9x + 8

4x=9x+854x = 9x + 8 - 5

4x=9x+34x = 9x + 3

4x9x=34x - 9x = 3

5x=3-5x = 3

x=53x = \dfrac{-5}{3}

Q20

Find the measures of the angles of these triangles.

Q21

Write 4 equations whose solution is u=6u = 6.

Q22

The Bakhshālī Manuscript (300 CE) mentions the following problem. The amount given to the first person is not known. The second person is given twice as much as the first. The third person is given thrice as much as the second; and the fourth person four times as much as the third. The total amount distributed is 132. What is the amount given to the first person?

Q23

The height of a giraffe is two and a half metres more than half its height. How tall is the giraffe?

Q24

Two separate figures are given below. Each figure shows the first few positions in a sequence of arrangements made with sticks. Identify the pattern and answer the following questions for each figure:

(a) How many squares are in position number 11 of the sequence? (b) How many sticks are needed to make the arrangement in position number 11 of the sequence? (c) Can an arrangement in this sequence be made using exactly 85 sticks? If yes, which position number will it correspond to? (d) Can an arrangement in this sequence be made using exactly 150 sticks? If yes, which position number will it correspond to?

Q25

A number increased by 36 is equal to ten times itself. What is the number?

Q26

Solve these equations:

(a) 5(r+2)=105(r + 2) = 10

(b) 3(u+2)=2(u1)-3(u + 2) = 2(u - 1)

(c) 2(72n)=62(7 - 2n) = -6

(d) 2(x4)=162(x - 4) = -16

(e) 6(x1)=2(x1)46(x - 1) = 2(x - 1) - 4

(f) 37s=73s3 - 7s = 7 - 3s

(g) 2x+1=6(2x3)2x + 1 = 6 - (2x - 3)

(h) 105x=3(x4)2(x7)10 - 5x = 3(x - 4) - 2(x - 7)

Q27

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