Julie started jogging from the farm to the nature reserve at a speed of 95 m/min at 7 a.m. Her sister started jogging from the farm later. They were beside each other at 7.40 a.m. and her sister reached the nature reserve at 8 a.m. while Julie was still 1900 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the farm?
1 h = 60 min

From 7 a.m. to 8 a.m. = 1 h
From 7.40 a.m. to 8 a.m. = 20 min

Distance between the farm and the nature reserve
= 60 x 95 + 1900
= 5700 + 1900
= 7600 m

Distance that Julie's sister had to cover to catch up with Julie
= 40 x 95
= 3800 m

Distance that Julie's sister travelled to the nature reserve after catching up with Julie
= 7600 - 3800
= 3800 m

Since Julie's sister jogged 3800 m in 40 min,
Julie's sister average speed
= 3800 ÷ 20
= 190 m/min

Duration that Julie's sister took to jog to the nature reserve
= 7600 ÷ 190
= 40 min

Time that Julie's sister left the farm:
40 min before 8 a.m. = 7.20 a.m.

Answer(s): 7.20 a.m.