Yen started jogging from the playground to the reservoir at a speed of 95 m/min at 9 a.m. Her sister started jogging from the playground later. They were beside each other at 9.40 a.m. and her sister reached the reservoir at 10 a.m. while Yen was still 1900 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the playground?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.40 a.m. to 10 a.m. = 20 min

Distance between the playground and the reservoir
= 60 x 95 + 1900
= 5700 + 1900
= 7600 m

Distance that Yen's sister had to cover to catch up with Yen
= 40 x 95
= 3800 m

Distance that Yen's sister travelled to the reservoir after catching up with Yen
= 7600 - 3800
= 3800 m

Since Yen's sister jogged 3800 m in 40 min,
Yen's sister average speed
= 3800 ÷ 20
= 190 m/min

Duration that Yen's sister took to jog to the reservoir
= 7600 ÷ 190
= 40 min

Time that Yen's sister left the playground:
40 min before 10 a.m. = 9.20 a.m.

Answer(s): 9.20 a.m.