Gwen started jogging from the school to the reservoir at a speed of 115 m/min at 7 a.m. Her sister started jogging from the school later. They were beside each other at 7.43 a.m. and her sister reached the reservoir at 8 a.m. while Gwen was still 1700 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the school?
1 h = 60 min

From 7 a.m. to 8 a.m. = 1 h
From 7.43 a.m. to 8 a.m. = 17 min

Distance between the school and the reservoir
= 60 x 115 + 1700
= 6900 + 1700
= 8600 m

Distance that Gwen's sister had to cover to catch up with Gwen
= 43 x 115
= 4945 m

Distance that Gwen's sister travelled to the reservoir after catching up with Gwen
= 8600 - 4945
= 3655 m

Since Gwen's sister jogged 3655 m in 43 min,
Gwen's sister average speed
= 3655 ÷ 17
= 215 m/min

Duration that Gwen's sister took to jog to the reservoir
= 8600 ÷ 215
= 40 min

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

Answer(s): 7.20 a.m.