Jaslyn started jogging from the bank to the reservoir at a speed of 105 m/min at 6 a.m. Her brother started jogging from the bank later. They were beside each other at 6.45 a.m. and her brother reached the reservoir at 7 a.m. while Jaslyn was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the bank?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.45 a.m. to 7 a.m. = 15 min

Distance between the bank and the reservoir
= 60 x 105 + 1800
= 6300 + 1800
= 8100 m

Distance that Jaslyn's brother had to cover to catch up with Jaslyn
= 45 x 105
= 4725 m

Distance that Jaslyn's brother travelled to the reservoir after catching up with Jaslyn
= 8100 - 4725
= 3375 m

Since Jaslyn's brother jogged 3375 m in 45 min,
Jaslyn's brother average speed
= 3375 ÷ 15
= 225 m/min

Duration that Jaslyn's brother took to jog to the reservoir
= 8100 ÷ 225
= 36 min

Time that Jaslyn's brother left the bank:
36 min before 7 a.m. = 6.24 a.m.

Answer(s): 6.24 a.m.