Winnie started jogging from the library to the reservoir at a speed of 105 m/min at 6 a.m. Her sister started jogging from the library later. They were beside each other at 6.41 a.m. and her sister reached the reservoir at 7 a.m. while Winnie was still 1900 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the library?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.41 a.m. to 7 a.m. = 19 min

Distance between the library and the reservoir
= 60 x 105 + 1900
= 6300 + 1900
= 8200 m

Distance that Winnie's sister had to cover to catch up with Winnie
= 41 x 105
= 4305 m

Distance that Winnie's sister travelled to the reservoir after catching up with Winnie
= 8200 - 4305
= 3895 m

Since Winnie's sister jogged 3895 m in 41 min,
Winnie's sister average speed
= 3895 ÷ 19
= 205 m/min

Duration that Winnie's sister took to jog to the reservoir
= 8200 ÷ 205
= 40 min

Time that Winnie's sister left the library:
40 min before 7 a.m. = 6.20 a.m.

Answer(s): 6.20 a.m.