Two numbers satisfy $x + y = 14$ and $x - y = 4$. What is $xy$?

Shortcut: $xy = \frac{(x + y)^2 - (x - y)^2}{4} = \frac{196 - 16}{4} = 45$. Forgetting the division by 4 gives 180; 56 is just $14 \times 4$.