Barbara started jogging from the florist to the reservoir at a speed of 105 m/min at 8 a.m. Her brother started jogging from the florist later. They were beside each other at 8.40 a.m. and her brother reached the reservoir at 9 a.m. while Barbara was still 1900 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the florist?
1 h = 60 min

From 8 a.m. to 9 a.m. = 1 h
From 8.40 a.m. to 9 a.m. = 20 min

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

Distance that Barbara's brother had to cover to catch up with Barbara
= 40 x 105
= 4200 m

Distance that Barbara's brother travelled to the reservoir after catching up with Barbara
= 8200 - 4200
= 4000 m

Since Barbara's brother jogged 4000 m in 40 min,
Barbara's brother average speed
= 4000 ÷ 20
= 200 m/min

Duration that Barbara's brother took to jog to the reservoir
= 8200 ÷ 200
= 41 min

Time that Barbara's brother left the florist:
41 min before 9 a.m. = 8.19 a.m.

Answer(s): 8.19 a.m.