Mary started jogging from the museum to the reservoir at a speed of 110 m/min at 9 a.m. Her cousin started jogging from the museum later. They were beside each other at 9.42 a.m. and her cousin reached the reservoir at 10 a.m. while Mary was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her cousin leave the museum?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.42 a.m. to 10 a.m. = 18 min

Distance between the museum and the reservoir
= 60 x 110 + 1800
= 6600 + 1800
= 8400 m

Distance that Mary's cousin had to cover to catch up with Mary
= 42 x 110
= 4620 m

Distance that Mary's cousin travelled to the reservoir after catching up with Mary
= 8400 - 4620
= 3780 m

Since Mary's cousin jogged 3780 m in 42 min,
Mary's cousin average speed
= 3780 ÷ 18
= 210 m/min

Duration that Mary's cousin took to jog to the reservoir
= 8400 ÷ 210
= 40 min

Time that Mary's cousin left the museum:
40 min before 10 a.m. = 9.20 a.m.

Answer(s): 9.20 a.m.