Risa started jogging from the mosque to the reservoir at a speed of 108 m/min at 9 a.m. Her brother started jogging from the mosque later. They were beside each other at 9.35 a.m. and her brother reached the reservoir at 10 a.m. while Risa was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the mosque?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.35 a.m. to 10 a.m. = 25 min

Distance between the mosque and the reservoir
= 60 x 108 + 1800
= 6480 + 1800
= 8280 m

Distance that Risa's brother had to cover to catch up with Risa
= 35 x 108
= 3780 m

Distance that Risa's brother travelled to the reservoir after catching up with Risa
= 8280 - 3780
= 4500 m

Since Risa's brother jogged 4500 m in 35 min,
Risa's brother average speed
= 4500 ÷ 25
= 180 m/min

Duration that Risa's brother took to jog to the reservoir
= 8280 ÷ 180
= 46 min

Time that Risa's brother left the mosque:
46 min before 10 a.m. = 9.14 a.m.

Answer(s): 9.14 a.m.