Marion started jogging from the clinic to the reservoir at a speed of 115 m/min at 7 a.m. Her sister started jogging from the clinic later. They were beside each other at 7.43 a.m. and her sister reached the reservoir at 8 a.m. while Marion was still 1700 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the clinic?
1 h = 60 min

From 7 a.m. to 8 a.m. = 1 h
From 7.43 a.m. to 8 a.m. = 17 min

Distance between the clinic and the reservoir
= 60 x 115 + 1700
= 6900 + 1700
= 8600 m

Distance that Marion's sister had to cover to catch up with Marion
= 43 x 115
= 4945 m

Distance that Marion's sister travelled to the reservoir after catching up with Marion
= 8600 - 4945
= 3655 m

Since Marion's sister jogged 3655 m in 43 min,
Marion's sister average speed
= 3655 ÷ 17
= 215 m/min

Duration that Marion's sister took to jog to the reservoir
= 8600 ÷ 215
= 40 min

Time that Marion's sister left the clinic:
40 min before 8 a.m. = 7.20 a.m.

Answer(s): 7.20 a.m.