A piece of metal is heated to 428 °C and left to cool. Its temperature falls at 23 °C per minute for the first 3 minutes, 15 °C per minute for the next 15 minutes, and then 8 °C per minute until it reaches room temperature of 25 °C. Find the temperature of the metal after 9 minutes.

Correct answer: 269. After 3 minutes: 428 − 3 × 23 = 359 °C. The next 6 minutes (to minute 9) fall by 6 × 15 = 90 °C, giving 269 °C.

• 258: 258 °C does not follow from the rates given.
• 273: 273 °C does not follow from the rates given.
• None of these: “None of these” is incorrect because 269 °C is correct.