In a proportion, the 1st, 2nd and 4th terms are 51, 68 and 108 respectively. Find the 3rd term.

The correct answer is 81

Let the 3rd term be $x$.

Thus we have, 51:68 :: $x$:108

⇒ $\dfrac{51}{68}$ = $\dfrac{x}{108}$

⇒ $\dfrac{3}{4}$ = $\dfrac{x}{108}$

∴ $x$ = $\dfrac{108 × 3}{4}$ = 81

Hence the 3rd term is 81.