Wendy started jogging from the clinic to the nature reserve at a speed of 105 m/min at 5 a.m. Her brother started jogging from the clinic later. They were beside each other at 5.40 a.m. and her brother reached the nature reserve at 6 a.m. while Wendy was still 1400 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the clinic?
1 h = 60 min

From 5 a.m. to 6 a.m. = 1 h
From 5.40 a.m. to 6 a.m. = 20 min

Distance between the clinic and the nature reserve
= 60 x 105 + 1400
= 6300 + 1400
= 7700 m

Distance that Wendy's brother had to cover to catch up with Wendy
= 40 x 105
= 4200 m

Distance that Wendy's brother travelled to the nature reserve after catching up with Wendy
= 7700 - 4200
= 3500 m

Since Wendy's brother jogged 3500 m in 40 min,
Wendy's brother average speed
= 3500 ÷ 20
= 175 m/min

Duration that Wendy's brother took to jog to the nature reserve
= 7700 ÷ 175
= 44 min

Time that Wendy's brother left the clinic:
44 min before 6 a.m. = 5.16 a.m.

Answer(s): 5.16 a.m.