Yen started jogging from the hospital to the nature reserve at a speed of 96 m/min at 8 a.m. Her sister started jogging from the hospital later. They were beside each other at 8.35 a.m. and her sister reached the nature reserve at 9 a.m. while Yen was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the hospital?
1 h = 60 min

From 8 a.m. to 9 a.m. = 1 h
From 8.35 a.m. to 9 a.m. = 25 min

Distance between the hospital and the nature reserve
= 60 x 96 + 1800
= 5760 + 1800
= 7560 m

Distance that Yen's sister had to cover to catch up with Yen
= 35 x 96
= 3360 m

Distance that Yen's sister travelled to the nature reserve after catching up with Yen
= 7560 - 3360
= 4200 m

Since Yen's sister jogged 4200 m in 35 min,
Yen's sister average speed
= 4200 ÷ 25
= 168 m/min

Duration that Yen's sister took to jog to the nature reserve
= 7560 ÷ 168
= 45 min

Time that Yen's sister left the hospital:
45 min before 9 a.m. = 8.15 a.m.

Answer(s): 8.15 a.m.