Raeann started jogging from the bistro to the reservoir at a speed of 100 m/min at 5 a.m. Her sister started jogging from the bistro later. They were beside each other at 5.42 a.m. and her sister reached the reservoir at 6 a.m. while Raeann was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the bistro?
1 h = 60 min

From 5 a.m. to 6 a.m. = 1 h
From 5.42 a.m. to 6 a.m. = 18 min

Distance between the bistro and the reservoir
= 60 x 100 + 1800
= 6000 + 1800
= 7800 m

Distance that Raeann's sister had to cover to catch up with Raeann
= 42 x 100
= 4200 m

Distance that Raeann's sister travelled to the reservoir after catching up with Raeann
= 7800 - 4200
= 3600 m

Since Raeann's sister jogged 3600 m in 42 min,
Raeann's sister average speed
= 3600 ÷ 18
= 200 m/min

Duration that Raeann's sister took to jog to the reservoir
= 7800 ÷ 200
= 39 min

Time that Raeann's sister left the bistro:
39 min before 6 a.m. = 5.21 a.m.

Answer(s): 5.21 a.m.