Jen started jogging from the grocery store to the nature reserve at a speed of 115 m/min at 9 a.m. Her cousin started jogging from the grocery store later. They were beside each other at 9.40 a.m. and her cousin reached the nature reserve at 10 a.m. while Jen was still 1700 m away. If both of them travelled at a constant speed throughout the journey, what time did her cousin leave the grocery store?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.40 a.m. to 10 a.m. = 20 min

Distance between the grocery store and the nature reserve
= 60 x 115 + 1700
= 6900 + 1700
= 8600 m

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

Distance that Jen's cousin travelled to the nature reserve after catching up with Jen
= 8600 - 4600
= 4000 m

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

Duration that Jen's cousin took to jog to the nature reserve
= 8600 ÷ 200
= 43 min

Time that Jen's cousin left the grocery store:
43 min before 10 a.m. = 9.17 a.m.

Answer(s): 9.17 a.m.