Rachel started jogging from the grocery store to the reservoir at a speed of 100 m/min at 6 a.m. Her brother started jogging from the grocery store later. They were beside each other at 6.45 a.m. and her brother reached the reservoir at 7 a.m. while Rachel was still 1200 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the grocery store?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.45 a.m. to 7 a.m. = 15 min

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

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

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

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

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

Time that Rachel's brother left the grocery store:
40 min before 7 a.m. = 6.20 a.m.

Answer(s): 6.20 a.m.