Question

Find the third proportional to the following:

20, 15

Answer 1

Let x be the third proportional to 20, 15.

Third proportional to 20, 15 is the same thing as fourth proportional to 20, 15, 15, then

20 : 15 :: 15 : x

⇒ `20/15 = 15/x`

⇒ 20 × x = 15 × 15

⇒ 20x = 225

⇒ x = `225/20`

⇒ x = 11.25