Jen started jogging from the bakery to the reservoir at a speed of 100 m/min at 9 a.m. Her sister started jogging from the bakery later. They were beside each other at 9.42 a.m. and her sister reached the reservoir at 10 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 bakery?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.42 a.m. to 10 a.m. = 18 min

Distance between the bakery and the reservoir
= 60 x 100 + 1800
= 6000 + 1800
= 7800 m

Distance that Jen's sister had to cover to catch up with Jen
= 42 x 100
= 4200 m

Distance that Jen's sister travelled to the reservoir after catching up with Jen
= 7800 - 4200
= 3600 m

Since Jen's sister jogged 3600 m in 42 min,
Jen's sister average speed
= 3600 ÷ 18
= 200 m/min

Duration that Jen's sister took to jog to the reservoir
= 7800 ÷ 200
= 39 min

Time that Jen's sister left the bakery:
39 min before 10 a.m. = 9.21 a.m.

Answer(s): 9.21 a.m.