Min started jogging from the temple to the reservoir at a speed of 110 m/min at 6 a.m. Her brother started jogging from the temple later. They were beside each other at 6.42 a.m. and her brother reached the reservoir at 7 a.m. while Min was still 1800 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 6 a.m. to 7 a.m. = 1 h
From 6.42 a.m. to 7 a.m. = 18 min

Distance between the temple and the reservoir
= 60 x 110 + 1800
= 6600 + 1800
= 8400 m

Distance that Min's brother had to cover to catch up with Min
= 42 x 110
= 4620 m

Distance that Min's brother travelled to the reservoir after catching up with Min
= 8400 - 4620
= 3780 m

Since Min's brother jogged 3780 m in 42 min,
Min's brother average speed
= 3780 ÷ 18
= 210 m/min

Duration that Min's brother took to jog to the reservoir
= 8400 ÷ 210
= 40 min

Time that Min's brother left the temple:
40 min before 7 a.m. = 6.20 a.m.

Answer(s): 6.20 a.m.