Solution: Original area: \( \frac{1}{2} \times 15 \times 20 = 150 \) km². New shorter leg: \( 15 + 5 = 20 \) km. New area: \( \frac{1}{2} \times 20 \times 20 = 200 \) km². The increase is \( 200 - 150 = 50 \) km². \boxed{50}