If x - y = 1, x + y = 17, then xy = ?

Answer: Option B

$$\begin{array}{rl}& \sqrt{x}-\sqrt{y}=1......(i)\\ & \sqrt{x}+\sqrt{y}=17.....(ii)\\ & \text{From equation (i) and (ii)}\\ & \sqrt{x}=9,\sqrt{y}=8\\ & \text{So, }\sqrt{xy}=9\times 8=72\end{array}$$