If 36 : 81 :: X : 63, what is the value of X?

In proportion: a : b :: c : d

⇒ Product of outer values = Product of inner values.

Therefore, a × d = b × c

Now, ATQ

⇒ 36 : 81 :: X : 63

⇒ 36 × 63 = 81 × X

⇒ X = \(\frac{36\; \times\; 63}{81}\) = 28