Wendy started jogging from the library to the nature reserve at a speed of 96 m/min at 9 a.m. Her sister started jogging from the library later. They were beside each other at 9.45 a.m. and her sister reached the nature reserve at 10 a.m. while Wendy was still 1800 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 9 a.m. to 10 a.m. = 1 h
From 9.45 a.m. to 10 a.m. = 15 min

Distance between the library and the nature reserve
= 60 x 96 + 1800
= 5760 + 1800
= 7560 m

Distance that Wendy's sister had to cover to catch up with Wendy
= 45 x 96
= 4320 m

Distance that Wendy's sister travelled to the nature reserve after catching up with Wendy
= 7560 - 4320
= 3240 m

Since Wendy's sister jogged 3240 m in 45 min,
Wendy's sister average speed
= 3240 ÷ 15
= 216 m/min

Duration that Wendy's sister took to jog to the nature reserve
= 7560 ÷ 216
= 35 min

Time that Wendy's sister left the library:
35 min before 10 a.m. = 9.25 a.m.

Answer(s): 9.25 a.m.