Roshel started jogging from the church to the nature reserve at a speed of 100 m/min at 9 a.m. Her cousin started jogging from the church later. They were beside each other at 9.30 a.m. and her cousin reached the nature reserve at 10 a.m. while Roshel was still 1500 m away. If both of them travelled at a constant speed throughout the journey, what time did her cousin leave the church?
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 church and the nature reserve
= 60 x 100 + 1500
= 6000 + 1500
= 7500 m

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

Distance that Roshel's cousin travelled to the nature reserve after catching up with Roshel
= 7500 - 3000
= 4500 m

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

Duration that Roshel's cousin took to jog to the nature reserve
= 7500 ÷ 150
= 50 min

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

Answer(s): 9.10 a.m.