Cathy started jogging from the mosque to the garden at a speed of 115 m/min at 8 a.m. Her brother started jogging from the mosque later. They were beside each other at 8.40 a.m. and her brother reached the garden at 9 a.m. while Cathy was still 1700 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the mosque?
1 h = 60 min

From 8 a.m. to 9 a.m. = 1 h
From 8.40 a.m. to 9 a.m. = 20 min

Distance between the mosque and the garden
= 60 x 115 + 1700
= 6900 + 1700
= 8600 m

Distance that Cathy's brother had to cover to catch up with Cathy
= 40 x 115
= 4600 m

Distance that Cathy's brother travelled to the garden after catching up with Cathy
= 8600 - 4600
= 4000 m

Since Cathy's brother jogged 4000 m in 40 min,
Cathy's brother average speed
= 4000 ÷ 20
= 200 m/min

Duration that Cathy's brother took to jog to the garden
= 8600 ÷ 200
= 43 min

Time that Cathy's brother left the mosque:
43 min before 9 a.m. = 8.17 a.m.

Answer(s): 8.17 a.m.