Penelope started jogging from the park to the nature reserve at a speed of 105 m/min at 9 a.m. Her brother started jogging from the park later. They were beside each other at 9.45 a.m. and her brother reached the nature reserve at 10 a.m. while Penelope was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the park?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.45 a.m. to 10 a.m. = 15 min

Distance between the park and the nature reserve
= 60 x 105 + 1800
= 6300 + 1800
= 8100 m

Distance that Penelope's brother had to cover to catch up with Penelope
= 45 x 105
= 4725 m

Distance that Penelope's brother travelled to the nature reserve after catching up with Penelope
= 8100 - 4725
= 3375 m

Since Penelope's brother jogged 3375 m in 45 min,
Penelope's brother average speed
= 3375 ÷ 15
= 225 m/min

Duration that Penelope's brother took to jog to the nature reserve
= 8100 ÷ 225
= 36 min

Time that Penelope's brother left the park:
36 min before 10 a.m. = 9.24 a.m.

Answer(s): 9.24 a.m.