The energy stored in a spring is directly proportional to the square of its extension. When a spring is stretched by d cm, the energy stored is E joules. If the extension is decreased by 25%, find the percentage decrease in the energy stored.

Correct answer: 43.75%. New extension = 0.75d, so new energy = (0.75)² E = 0.5625E. The decrease is 1 − 0.5625 = 0.4375, i.e. 43.75%.

• 45.77%: 45.77% is a calculation error.
• 47.45%: 47.45% is a calculation error.
• 49.3%: 49.3% is a calculation error.