Gillian started jogging from the farm to the reservoir at a speed of 100 m/min at 9 a.m. Her cousin started jogging from the farm later. They were beside each other at 9.30 a.m. and her cousin reached the reservoir at 10 a.m. while Gillian was still 1500 m away. If both of them travelled at a constant speed throughout the journey, what time did her cousin leave the farm?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.30 a.m. to 10 a.m. = 30 min

Distance between the farm and the reservoir
= 60 x 100 + 1500
= 6000 + 1500
= 7500 m

Distance that Gillian's cousin had to cover to catch up with Gillian
= 30 x 100
= 3000 m

Distance that Gillian's cousin travelled to the reservoir after catching up with Gillian
= 7500 - 3000
= 4500 m

Since Gillian's cousin jogged 4500 m in 30 min,
Gillian's cousin average speed
= 4500 ÷ 30
= 150 m/min

Duration that Gillian's cousin took to jog to the reservoir
= 7500 ÷ 150
= 50 min

Time that Gillian's cousin left the farm:
50 min before 10 a.m. = 9.10 a.m.

Answer(s): 9.10 a.m.