Finding the Unknown | FIO

Question 19

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}

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Solution
Understand the Question
  • To solve linear equations correctly, keep these fundamental algebraic rules in mind:
    • Uniform Operation: When dividing or multiplying an equation by a number, every term on both sides must be divided or multiplied.
    • Transposition Rule: When moving a term from one side of the equals sign to the other, its sign must change (++ becomes -, and - becomes ++).
    • Isolating the Variable: When solving ax=bax = b, divide both sides by the coefficient aa to get x=bax = \dfrac{b}{a}.

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

x+9=11x + 9 = 11

x=119x = 11 - 9

x=2x = 2**

Step 1 · Identify the Mistake and Solve Correctly

Mistake: In the step x+9=11x + 9 = 11, only 6x6x and 6666 were divided by 66, but the term 99 was not divided. When dividing an equation by a number, every term must be divided.Diagram 1

Correct Solution: 6x+9=666x + 9 = 66

Subtract 99 from both sides

6x+99=6696x=57\begin{aligned} 6x + 9 - 9 &= 66 - 9 \\ 6x &= 57 \end{aligned}

Divide both sides by 66 x=576x = \dfrac{57}{6}

Simplify the fraction

x=57÷36÷3=192=912\begin{aligned} x &= \dfrac{57 \div 3}{6 \div 3} \\[0.6em] &= \dfrac{19}{2} \\[0.6em] &= 9\dfrac{1}{2} \end{aligned}
Answer

(a) Mistake: Only some terms were divided by 66. The correct value is x=192=912x = \dfrac{19}{2} = 9\dfrac{1}{2}.

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

7y+12=187y + 12 = 18

7y=67y = 6

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

Step 1 · Verify the Steps

Given equation 14y+24=3614y + 24 = 36

Dividing all terms on both sides by 22 7y+12=187y + 12 = 18

Subtracting 1212 from both sides

7y=18127y=6\begin{aligned} 7y &= 18 - 12 \\ 7y &= 6 \end{aligned}

Dividing both sides by 77 y=67y = \dfrac{6}{7}

All steps are algebraically correct. There is no error in this solution.

Answer

(b) There are no mistakes in the given steps. The answer is 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}**

Step 1 · Identify the Mistake and Solve Correctly

Mistakes:

  1. When moving 5-5 from LHS to RHS, its sign was not changed; it should become +5+5, not 5-5.
  2. In the last step, solving 5x=3-5x = 3 resulted in x=53x = \dfrac{-5}{3} instead of x=35=35x = \dfrac{3}{-5} = -\dfrac{3}{5}.

Correct Solution: 4x5=9x+84x - 5 = 9x + 8

Add 55 to both sides

4x5+5=9x+8+54x=9x+13\begin{aligned} 4x - 5 + 5 &= 9x + 8 + 5 \\ 4x &= 9x + 13 \end{aligned}

Subtract 9x9x from both sides

4x9x=9x+139x5x=13\begin{aligned} 4x - 9x &= 9x + 13 - 9x \\ -5x &= 13 \end{aligned}

Divide both sides by 5-5

x=135=135=235\begin{aligned} x &= \dfrac{13}{-5} \\[0.6em] &= -\dfrac{13}{5} \\[0.6em] &= -2\dfrac{3}{5} \end{aligned}
Answer

(c) Mistake: The sign of 5-5 was not changed to +5+5 when transposing. The correct value is x=135=235x = -\dfrac{13}{5} = -2\dfrac{3}{5}.

Common Mistakes
  • Partial Division: Dividing only selected terms in an equation rather than every single term on both sides.
  • Transposition Sign Error: Forgetting to flip the sign when moving a constant or variable across the equals sign (e.g. 5-5 must become +5+5).
  • Inverting the Final Fraction: Writing x=abx = \dfrac{a}{b} instead of x=bax = \dfrac{b}{a} when solving ax=bax = b.

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

Solve the equations to find a path from Start to the End. Show your work in the given boxes provided and colour your path as you proceed.

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