Shiyun started jogging from the florist to the reservoir at a speed of 115 m/min at 5 a.m. Her cousin started jogging from the florist later. They were beside each other at 5.40 a.m. and her cousin reached the reservoir at 6 a.m. while Shiyun was still 1700 m away. If both of them travelled at a constant speed throughout the journey, what time did her cousin leave the florist?
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 florist and the reservoir
= 60 x 115 + 1700
= 6900 + 1700
= 8600 m

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

Distance that Shiyun's cousin travelled to the reservoir after catching up with Shiyun
= 8600 - 4600
= 4000 m

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

Duration that Shiyun's cousin took to jog to the reservoir
= 8600 ÷ 200
= 43 min

Time that Shiyun's cousin left the florist:
43 min before 6 a.m. = 5.17 a.m.

Answer(s): 5.17 a.m.