Byomkesh then interrogates the guard to check whether he has monitored the vault right or not. The guard says that he monitors with full punctuality. Given the camera is on only for 30 second every 29 minute 30 second(cost cutting in bank: D). the power fluctuation in area is every 49 minutes after 9 and the power remains down for 1 minute. The first monitoring of camera started at 9:30. Help byomkesh in calculating if guard is telling the truth. Power went down first at 9:50.

The camera schedule starts at 9:30, then every 30 minutes: 10:00, 10:30, 11:00... Power goes down every 49 minutes after 9:00, first at 9:50 for 1 minute, then 10:39, 11:28, etc. At 10:00 (camera on), power is ON (from 9:51 to 10:38). At 10:30 (camera on), power is ON. At 10:39 (power off), camera would be OFF. At 11:00 (camera on), power is ON (from 10:40 to 11:27). However, by counting cycles, at 10:00 camera time would have power off from 9:49-9:50, but camera is only on 30 seconds starting at 9:59:30. By 11:38, the pattern shows camera was on during power outages, proving the guard lied.