Question

A watch which gains uniformly is 5 minutes slow at 10 o'clock in the morning on Monday and it is 5 min 48 seconds ahead at 10 p.m. on the following Monday. When was the clock showing the correct time?

• A. 9 p.m. on Thursday
• B. 20 minutes past 9 p.m. on Thursday
• C. 10 p.m. on Thursday
• D. 15 minutes past 9 p.m. on Thursday

Answer 1

The correct option is B 20 minutes past 9 p.m. on Thursday

So, the watch gains 648 (from 5 minutes behind to 5 minutes and 48 seconds ahead) seconds in 180 hours (10 a.m. in the morning of a day to 10 p.m. next week).
$\frac{648}{180}=3.6$ seconds per hour $\Rightarrow$ To gain 5 minutes $\frac{300}{3.6}=83\frac{1}{3}$ hours
83 hours and 20 minutes from 10 a.m. Monday is 20 minutes past 9 p.m. on Wednesday.