Question

What will be the first proportion when third proportion is 5. Second proportion is 4 more than the first proportion and fourth proportion is 7 more than the third proportion.

• A. 7
• B. 20
• C. $\frac{7}{20}$
• D. $\frac{20}{7}$

Answer 1

The correct option is D $\frac{20}{7}$

Given, third proportion = 5
So, fourth proportion = 5 + 7 = 12

Let, the first proportion = a
then, second proportion = a + 4

Since the ratio are in proportion, we have
a : (a + 4) :: 5 : 12

Product of means = Product of extremes
$\therefore$ (a + 4) $\times$ 5 = a $\times$ 12
$\Rightarrow$ 5a + 20 = 12a
$\Rightarrow$ 12a - 5a = 20
$\Rightarrow$ 7a = 20
$\Rightarrow$ a = $\frac{20}{7}$

So, Proportion =
$\frac{20}{7}$ : ($\frac{20}{7}$ + 4) :: 5 : 12