Jen started jogging from the school to the park at a speed of 96 m/min at 6 a.m. Her sister started jogging from the school later. They were beside each other at 6.35 a.m. and her sister reached the park at 7 a.m. while Jen was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the school?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.35 a.m. to 7 a.m. = 25 min

Distance between the school and the park
= 60 x 96 + 1800
= 5760 + 1800
= 7560 m

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

Distance that Jen's sister travelled to the park after catching up with Jen
= 7560 - 3360
= 4200 m

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

Duration that Jen's sister took to jog to the park
= 7560 ÷ 168
= 45 min

Time that Jen's sister left the school:
45 min before 7 a.m. = 6.15 a.m.

Answer(s): 6.15 a.m.