A pendulum clock having copper rod keeps correct time at  20oC.It gains 15 s per day if cooled to 0oC. The coefficient of linear expansion of copper is

T20−T0T0=12×α×20=10αHere T20 is the correct time period. The time period at 0oC is smaller so that the clock runs fast. The time gained in 24h.              =24h×α×10⇒ 15s=24h×α×10α=15s86400s×10∘C=1.7×10−5/∘C