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 = \frac{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 = \frac{-5}{3}

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Solution

We will check each equation step by step to find any errors and then solve them correctly.

Step 1 — Equation (a) Analysis

Let us look at the first equation. The given equation is 6x+9=666x + 9 = 66. The next step shown is x+9=11x + 9 = 11. This step is wrong. To get xx from 6x6x, we divide by 6. But we must divide all terms by 6. Here, only 6x6x and 6666 were divided by 6. The number 9 was not divided by 6. So, this is a mistake.

Let us solve it correctly. First, we move the number 9 to the other side. We subtract 9 from both sides of the equation. 6x+99=6696x + 9 - 9 = 66 - 9 6x=576x = 57 Now, we want to find xx. We divide both sides by 6. x=576x = \frac{57}{6} We can simplify this fraction. Both 57 and 6 can be divided by 3. x=57÷36÷3x = \frac{57 \div 3}{6 \div 3} x=192x = \frac{19}{2} We can write this as a mixed number. x=912x = 9 \frac{1}{2}

x=912\boxed{x = 9 \frac{1}{2}}

Diagram 1

Step 2 — Equation (b) Analysis

Let us look at the second equation. The given equation is 14y+24=3614y + 24 = 36. The next step shown is 7y+12=187y + 12 = 18. This step is correct. All terms were divided by 2. Then, 12 was subtracted from both sides. 7y=18127y = 18 - 12 7y=67y = 6 Finally, both sides were divided by 7. y=67y = \frac{6}{7} All steps are correct. There is no error in this solution.

y=67\boxed{y = \frac{6}{7}}

Step 3 — Equation (c) Analysis

Let us look at the third equation. The given equation is 4x5=9x+84x - 5 = 9x + 8. The next step shown is 4x=9x+854x = 9x + 8 - 5. This step is wrong. When we move -5 from the left side to the right side, its sign must change. It should become +5, not -5. So, this is a mistake.

Let us solve it correctly. First, we move the number -5 to the other side. We add 5 to both sides of the equation. 4x5+5=9x+8+54x - 5 + 5 = 9x + 8 + 5 4x=9x+134x = 9x + 13 Now, we want to gather all xx terms on one side. We subtract 9x9x from both sides. 4x9x=9x+139x4x - 9x = 9x + 13 - 9x 5x=13-5x = 13 Finally, we want to find xx. We divide both sides by -5. x=135x = \frac{13}{-5} We can write this as a mixed number. x=235x = -2 \frac{3}{5}

x=235\boxed{x = -2 \frac{3}{5}}

Answer

(a) The mistake was in dividing only some terms by 6. The correct answer is x=912x = 9 \frac{1}{2}. (b) There were no mistakes in the given steps. The answer is y=67y = \frac{6}{7}. (c) The mistake was in changing the sign of -5 incorrectly when moving it. The correct answer is x=235x = -2 \frac{3}{5}.

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\frac{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 = \frac{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 = \frac{-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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