Jane started jogging from the bistro to the reservoir at a speed of 95 m/min at 6 a.m. Her brother started jogging from the bistro later. They were beside each other at 6.40 a.m. and her brother reached the reservoir at 7 a.m. while Jane was still 1900 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the bistro?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.40 a.m. to 7 a.m. = 20 min

Distance between the bistro and the reservoir
= 60 x 95 + 1900
= 5700 + 1900
= 7600 m

Distance that Jane's brother had to cover to catch up with Jane
= 40 x 95
= 3800 m

Distance that Jane's brother travelled to the reservoir after catching up with Jane
= 7600 - 3800
= 3800 m

Since Jane's brother jogged 3800 m in 40 min,
Jane's brother average speed
= 3800 ÷ 20
= 190 m/min

Duration that Jane's brother took to jog to the reservoir
= 7600 ÷ 190
= 40 min

Time that Jane's brother left the bistro:
40 min before 7 a.m. = 6.20 a.m.

Answer(s): 6.20 a.m.