If a : b :: b : c; c = 4a and b = 18, find the positive value of c.

Correct option is A

Given:

a : b = b : c (a proportion relation)

c = 4a

b = 18

Solution:

$$4a^2 = b^2\\ \ \\4a^2 = 18^2\\ \ \\4a^2 = 324\\ \ \\a^2 = \frac{324}{4} = 81$$ 

a = 9 

c = 4a = 4 × 9 = 36