Yen started jogging from the farm to the park at a speed of 115 m/min at 9 a.m. Her sister started jogging from the farm later. They were beside each other at 9.35 a.m. and her sister reached the park at 10 a.m. while Yen was still 1500 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the farm?
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 farm and the park
= 60 x 115 + 1500
= 6900 + 1500
= 8400 m

Distance that Yen's sister had to cover to catch up with Yen
= 35 x 115
= 4025 m

Distance that Yen's sister travelled to the park after catching up with Yen
= 8400 - 4025
= 4375 m

Since Yen's sister jogged 4375 m in 35 min,
Yen's sister average speed
= 4375 ÷ 25
= 175 m/min

Duration that Yen's sister took to jog to the park
= 8400 ÷ 175
= 48 min

Time that Yen's sister left the farm:
48 min before 10 a.m. = 9.12 a.m.

Answer(s): 9.12 a.m.