Marion started jogging from the grocery store to the park at a speed of 105 m/min at 9 a.m. Her brother started jogging from the grocery store later. They were beside each other at 9.36 a.m. and her brother reached the park at 10 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 brother leave the grocery store?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.36 a.m. to 10 a.m. = 24 min

Distance between the grocery store and the park
= 60 x 105 + 1800
= 6300 + 1800
= 8100 m

Distance that Marion's brother had to cover to catch up with Marion
= 36 x 105
= 3780 m

Distance that Marion's brother travelled to the park after catching up with Marion
= 8100 - 3780
= 4320 m

Since Marion's brother jogged 4320 m in 36 min,
Marion's brother average speed
= 4320 ÷ 24
= 180 m/min

Duration that Marion's brother took to jog to the park
= 8100 ÷ 180
= 45 min

Time that Marion's brother left the grocery store:
45 min before 10 a.m. = 9.15 a.m.

Answer(s): 9.15 a.m.