x is the 4th proportional to 12, 16 and 5; and 20, y, 15,21 are in pr…

12 : 16 :: 5 : x,                 20 : y :: 15 : 21

⇒ 12x = 16 x 5               ⇒ 15y = 20 x 21   

⇒ 12x = 80                    ⇒ 15y = 420

⇒ x = \(\frac{40}{6}\)   ⇒ y = 28

⇒ (6x - y) = 6 x \(\frac{40}{6}\) - 28

⇒ 40 - 28 = 12.