Joelle started jogging from the mosque to the reservoir at a speed of 100 m/min at 7 a.m. Her sister started jogging from the mosque later. They were beside each other at 7.42 a.m. and her sister reached the reservoir at 8 a.m. while Joelle was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the mosque?
1 h = 60 min

From 7 a.m. to 8 a.m. = 1 h
From 7.42 a.m. to 8 a.m. = 18 min

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

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

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

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

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

Time that Joelle's sister left the mosque:
39 min before 8 a.m. = 7.21 a.m.

Answer(s): 7.21 a.m.