Erika started jogging from the hypermart to the reservoir at a speed of 105 m/min at 5 a.m. Her brother started jogging from the hypermart later. They were beside each other at 5.36 a.m. and her brother reached the reservoir at 6 a.m. while Erika was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the hypermart?
1 h = 60 min

From 5 a.m. to 6 a.m. = 1 h
From 5.36 a.m. to 6 a.m. = 24 min

Distance between the hypermart and the reservoir
= 60 x 105 + 1800
= 6300 + 1800
= 8100 m

Distance that Erika's brother had to cover to catch up with Erika
= 36 x 105
= 3780 m

Distance that Erika's brother travelled to the reservoir after catching up with Erika
= 8100 - 3780
= 4320 m

Since Erika's brother jogged 4320 m in 36 min,
Erika's brother average speed
= 4320 ÷ 24
= 180 m/min

Duration that Erika's brother took to jog to the reservoir
= 8100 ÷ 180
= 45 min

Time that Erika's brother left the hypermart:
45 min before 6 a.m. = 5.15 a.m.

Answer(s): 5.15 a.m.