Jean started jogging from the temple to the reservoir at a speed of 100 m/min at 9 a.m. Her brother started jogging from the temple later. They were beside each other at 9.45 a.m. and her brother reached the reservoir at 10 a.m. while Jean was still 1200 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the temple?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.45 a.m. to 10 a.m. = 15 min

Distance between the temple and the reservoir
= 60 x 100 + 1200
= 6000 + 1200
= 7200 m

Distance that Jean's brother had to cover to catch up with Jean
= 45 x 100
= 4500 m

Distance that Jean's brother travelled to the reservoir after catching up with Jean
= 7200 - 4500
= 2700 m

Since Jean's brother jogged 2700 m in 45 min,
Jean's brother average speed
= 2700 ÷ 15
= 180 m/min

Duration that Jean's brother took to jog to the reservoir
= 7200 ÷ 180
= 40 min

Time that Jean's brother left the temple:
40 min before 10 a.m. = 9.20 a.m.

Answer(s): 9.20 a.m.