Nora started jogging from the boutique to the park at a speed of 100 m/min at 9 a.m. Her brother started jogging from the boutique later. They were beside each other at 9.36 a.m. and her brother reached the park at 10 a.m. while Nora was still 1200 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the boutique?
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 boutique and the park
= 60 x 100 + 1200
= 6000 + 1200
= 7200 m

Distance that Nora's brother had to cover to catch up with Nora
= 36 x 100
= 3600 m

Distance that Nora's brother travelled to the park after catching up with Nora
= 7200 - 3600
= 3600 m

Since Nora's brother jogged 3600 m in 36 min,
Nora's brother average speed
= 3600 ÷ 24
= 150 m/min

Duration that Nora's brother took to jog to the park
= 7200 ÷ 150
= 48 min

Time that Nora's brother left the boutique:
48 min before 10 a.m. = 9.12 a.m.

Answer(s): 9.12 a.m.