UPSC 2015 CDS I Math Paper

Duration of time between the first time and the last time the light was seen
= 3 hr 17 min 49 sec - 1 hr 54 min 50 sec
=11869‌sec‌–‌6890‌sec
=4979‌sec
It is given that the light was seenregularly at an interval of 13 seconds.
Therefore, the number of times the light was seen =

4979

13

+1
=383+1=384