# Maharashtra Board 8th Class Maths Practice Set 7.2 Solutions Chapter 7 Variation

Balbharti Maharashtra State Board Class 8 Maths Solutions covers the Practice Set 7.2 8th Std Maths Answers Solutions Chapter 7 Variation.

## Practice Set 7.2 8th Std Maths Answers Chapter 7 Variation

Question 1.
The information about number of workers and number of days to complete a work is given in the following table. Complete the table.

 Number of workers 30 20 __ 10 __ Days 6 9 12 36

Solution:
Let, n represent the number of workers and d represent the number of days required to complete a work.
Since, number of workers and number of days to complete a work are in inverse poportion.
∴ $$\mathbf{n} \propto \frac{1}{\mathrm{d}}$$
∴ $$\mathrm{n}=\mathrm{k} \times \frac{1}{\mathrm{d}}$$
where k is the constant of variation.
∴ n × d = k …(i)

i. When n = 30, d = 6
∴ Substituting n = 30 and d = 6 in (i), we get
n × d = k
∴ 30 × 6 = k
∴ k = 180
Substituting k = 180 in (i), we get
∴ n × d = k
∴ n × d = 180 …(ii)
This is the equation of variation

ii. When d = 12, n = 7
∴ Substituting d = 12 in (ii), we get
n × d = 180
∴ n × 12 = 180
∴ n = $$\frac { 180 }{ 12 }$$
∴ n = 15

iii. When n = 10, d = ?
∴ Substituting n = 10 in (ii), we get
n × d = 180
10 × d = 180
∴ d = $$\frac { 180 }{ 10 }$$
∴ d = 18

iv. When d = 36, n = ?
∴ Substituting d = 36 in (ii), we get
n × d = 180
∴ n × 36 = 180
∴ n = $$\frac { 180 }{ 36 }$$
∴ n = 5

 Number of workers 30 20 15 10 5 Days 6 9 12 18 36

Question 2.
Find constant of variation and write equation of variation for every example given below:
i. $$p \propto \frac{1}{q}$$ ; if p = 15 then q = 4.
ii. $$z \propto \frac{1}{w}$$ ; when z = 2.5 then w = 24.
iii. $$s \propto \frac{1}{t^{2}}$$ ; if s = 4 then t = 5.
iv. $$x \propto \frac{1}{\sqrt{y}}$$ ; if x = 15 then y = 9.
Solution:
i. $$p \propto \frac{1}{q}$$ …[Given]
∴ p = k × $$\frac { 1 }{ q }$$
where, k is the constant of variation.
∴ p × q = k …(i)
When p = 15, q = 4
∴ Substituting p = 15 and q = 4 in (i), we get
p × q = k
∴ 15 × 4 = k
∴ k = 60
Substituting k = 60 in (i), we get
p × q = k
∴ p × q = 60
This is the equation of variation.
∴ The constant of variation is 60 and the equation of variation is pq = 60.

ii. $$z \propto \frac{1}{w}$$ …[Given]
∴ z = k × $$\frac { 1 }{ w }$$
where, k is the constant of variation,
∴ z × w = k …(i)
When z = 2.5, w = 24
∴ Substituting z = 2.5 and w = 24 in (i), we get
z × w = k
∴ 2.5 × 24 = k
∴ k = 60
Substituting k = 60 in (i), we get
z × w = k
∴ z × w = 60
This is the equation of variation.
∴ The constant of variation is 60 and the equation of variation is zw = 60.

iii. $$s \propto \frac{1}{t^{2}}$$ …[Given]
∴ $$s=k \times \frac{1}{t^{2}}$$
where, k is the constant of variation,
∴ s × t² = k …(i)
When s = 4, t = 5
∴ Substituting, s = 4 and t = 5 in (i), we get
s × t² = k
∴ 4 × (5)² = k
∴ k = 4 × 25
∴ k = 100
Substituting k = 100 in (i), we get
s × t² = k
∴ s × t² = 100
This is the equation of variation.
∴ The constant of variation is 100 and the equation of variation is st² = 100.

iv. $$x \propto \frac{1}{\sqrt{y}}$$ …[Given]
∴ $$x=\mathrm{k} \times \frac{1}{\sqrt{y}}$$
where, k is the constant of variation,
∴ x × √y = k …(i)
When x = 15, y = 9
∴ Substituting x = 15 and y = 9 in (i), we get
x × √y = k
∴ 15 × √9 = k
∴ k = 15 × 3
∴ k = 45
Substituting k = 45 in (i), we get
x × √y = k
∴ x × √y = 45 .
This is the equation of variation.
∴ The constant of variation is k = 45 and the equation of variation is x√y = 45.

Question 3.
The boxes are to be filled with apples in a heap. If 24 apples are put in a box then 27 boxes are needed. If 36 apples are filled in a box how many boxes will be needed?
Solution:
Let x represent the number of apples in each box and y represent the total number of boxes required.
The number of apples in each box are varying inversely with the total number of boxes.
∴ $$x \infty \frac{1}{y}$$
∴ $$x=k \times \frac{1}{y}$$
where, k is the constant of variation,
∴ x × y = k …(i)
If 24 apples are put in a box then 27 boxes are needed.
i.e., when x = 24, y = 27
∴ Substituting x = 24 and y = 27 in (i), we get
x × y = k
∴ 24 × 27 = k
∴ k = 648
Substituting k = 648 in (i), we get
x × y = k
∴ x × y = 648 …(ii)
This is the equation of variation.
Now, we have to find number of boxes needed
when, 36 apples are filled in each box.
i.e., when x = 36,y = ?
∴ Substituting x = 36 in (ii), we get
x × y = 648
∴ 36 × y = 648
∴ y = $$\frac { 648 }{ 36 }$$
∴ y = 18
∴ If 36 apples are filled in a box then 18 boxes are required.

Question 4.
Write the following statements using symbol of variation.

1. The wavelength of sound (l) and its frequency (f) are in inverse variation.
2. The intensity (I) of light varies inversely with the square of the distance (d) of a screen from the lamp.

Solution:

1. $$l \propto \frac{1}{\mathrm{f}}$$
2. $$\mathrm{I} \propto \frac{1}{\mathrm{d}^{2}}$$

Question 5.
$$x \propto \frac{1}{\sqrt{y}}$$ and when x = 40 then y = 16. If x = 10, find y.
Solution:
$$x \propto \frac{1}{\sqrt{y}}$$
∴ $$x=\mathrm{k} \times \frac{1}{\sqrt{y}}$$
where, k is the constant of variation.
∴ x × √y = k …(i)
When x = 40, y = 16
∴ Substituting x = 40 andy = 16 in (i), we get
x × √y = k
∴ 40 × √16 = k
∴ k = 40 × 4
∴ k = 160
Substituting k = 160 in (i), we get
x × √y = k
∴ x × √y = 160 …(ii)
This is the equation of variation.
When x = 10,y = ?
∴ Substituting, x = 10 in (ii), we get
x × √y = 160
∴ 10 × √y = 160
∴ √y = $$\frac { 160 }{ 10 }$$
∴ √y = 16
∴ y = 256 … [Squaring both sides]

Question 6.
x varies inversely as y, when x = 15 then y = 10, if x = 20, then y = ?
Solution:
Given that,
$$x \propto \frac{1}{\sqrt{y}}$$
∴ $$x=\mathrm{k} \times \frac{1}{\sqrt{y}}$$
where, k is the constant of variation.
∴ x × y = k …(i)
When x = 15, y = 10
∴ Substituting, x = 15 and y = 10 in (i), we get
x × y = k
∴ 15 × 10 = k
∴ k = 150
Substituting, k = 150 in (i), we get
x × y = k
∴ x × y = 150 …(ii)
This is the equation of variation.
When x = 20, y = ?
∴ substituting x = 20 in (ii), we get
x × y = 150
∴ 20 × y = 150
∴ y = $$\frac { 150 }{ 20 }$$
∴ y = 7.5