A mechanical grandfather clock is at present showing 7 hours 40 minutes 6 seconds. Assuming that it loses 4 seconds in every hour, what time will it show after exactly 61/2 hours ?

Answer: Option B

$$\begin{array}{rl}& \text{Time lost in }6\frac{1}{2}\text{ hours}\\ & =\left(6\frac{1}{2}\times 4\right)\sec \\ & =26\sec \end{array}$$
Correct time after $$\text{6}\frac{1}{2}$$ hours
= 7 hours 40 minutes 6 seconds + 6 hours 30 minutes
= 14 hours 10 minutes 6 seconds
Time show by the clock
= 14 hours 10 minutes 6 seconds - 26 sec
= 14 hours 9 minutes 40 seconds