UPSC 2016 CDS I Math Paper

At 5 o'clock, the clock strikes 5 times. There are 4 gaps between the numbers 1 to 5 on the clock.
Time taken by the clock to strike at 5 o'clock = 8 seconds
Time taken for each gap between 1 to 5=

8

4

=2seconds
At 10 o'clock, the clock strikes 10 times. There are 9 gaps between the numbers 1 to 10 on the clock.
∴ Time taken for each gap between 1 and 10=2×9=18seconds
Thus, the time taken by the clock tostrike at 10 o'clock is 18 seconds.