Marion started jogging from the boutique to the nature reserve at a speed of 105 m/min at 7 a.m. Her sister started jogging from the boutique later. They were beside each other at 7.40 a.m. and her sister reached the nature reserve at 8 a.m. while Marion was still 1400 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the boutique?
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 boutique and the nature reserve
= 60 x 105 + 1400
= 6300 + 1400
= 7700 m

Distance that Marion's sister had to cover to catch up with Marion
= 40 x 105
= 4200 m

Distance that Marion's sister travelled to the nature reserve after catching up with Marion
= 7700 - 4200
= 3500 m

Since Marion's sister jogged 3500 m in 40 min,
Marion's sister average speed
= 3500 ÷ 20
= 175 m/min

Duration that Marion's sister took to jog to the nature reserve
= 7700 ÷ 175
= 44 min

Time that Marion's sister left the boutique:
44 min before 8 a.m. = 7.16 a.m.

Answer(s): 7.16 a.m.