Cindy started jogging from the cinema to the reservoir at a speed of 108 m/min at 9 a.m. Her sister started jogging from the cinema later. They were beside each other at 9.35 a.m. and her sister reached the reservoir at 10 a.m. while Cindy was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the cinema?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.35 a.m. to 10 a.m. = 25 min

Distance between the cinema and the reservoir
= 60 x 108 + 1800
= 6480 + 1800
= 8280 m

Distance that Cindy's sister had to cover to catch up with Cindy
= 35 x 108
= 3780 m

Distance that Cindy's sister travelled to the reservoir after catching up with Cindy
= 8280 - 3780
= 4500 m

Since Cindy's sister jogged 4500 m in 35 min,
Cindy's sister average speed
= 4500 ÷ 25
= 180 m/min

Duration that Cindy's sister took to jog to the reservoir
= 8280 ÷ 180
= 46 min

Time that Cindy's sister left the cinema:
46 min before 10 a.m. = 9.14 a.m.

Answer(s): 9.14 a.m.