Joelle started jogging from the library to the park at a speed of 100 m/min at 9 a.m. Her cousin started jogging from the library later. They were beside each other at 9.40 a.m. and her cousin reached the park at 10 a.m. while Joelle was still 1200 m away. If both of them travelled at a constant speed throughout the journey, what time did her cousin leave the library?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.40 a.m. to 10 a.m. = 20 min

Distance between the library and the park
= 60 x 100 + 1200
= 6000 + 1200
= 7200 m

Distance that Joelle's cousin had to cover to catch up with Joelle
= 40 x 100
= 4000 m

Distance that Joelle's cousin travelled to the park after catching up with Joelle
= 7200 - 4000
= 3200 m

Since Joelle's cousin jogged 3200 m in 40 min,
Joelle's cousin average speed
= 3200 ÷ 20
= 160 m/min

Duration that Joelle's cousin took to jog to the park
= 7200 ÷ 160
= 45 min

Time that Joelle's cousin left the library:
45 min before 10 a.m. = 9.15 a.m.

Answer(s): 9.15 a.m.