Marion started jogging from the mall to the garden at a speed of 100 m/min at 5 a.m. Her sister started jogging from the mall later. They were beside each other at 5.42 a.m. and her sister reached the garden at 6 a.m. while Marion was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the mall?
1 h = 60 min

From 5 a.m. to 6 a.m. = 1 h
From 5.42 a.m. to 6 a.m. = 18 min

Distance between the mall and the garden
= 60 x 100 + 1800
= 6000 + 1800
= 7800 m

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

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

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

Duration that Marion's sister took to jog to the garden
= 7800 ÷ 200
= 39 min

Time that Marion's sister left the mall:
39 min before 6 a.m. = 5.21 a.m.

Answer(s): 5.21 a.m.