Dana started jogging from the cinema to the reservoir at a speed of 102 m/min at 9 a.m. Her brother started jogging from the cinema later. They were beside each other at 9.35 a.m. and her brother reached the reservoir at 10 a.m. while Dana was still 1700 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the cinema?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.35 a.m. to 10 a.m. = 25 min

Distance between the cinema and the reservoir
= 60 x 102 + 1700
= 6120 + 1700
= 7820 m

Distance that Dana's brother had to cover to catch up with Dana
= 35 x 102
= 3570 m

Distance that Dana's brother travelled to the reservoir after catching up with Dana
= 7820 - 3570
= 4250 m

Since Dana's brother jogged 4250 m in 35 min,
Dana's brother average speed
= 4250 ÷ 25
= 170 m/min

Duration that Dana's brother took to jog to the reservoir
= 7820 ÷ 170
= 46 min

Time that Dana's brother left the cinema:
46 min before 10 a.m. = 9.14 a.m.

Answer(s): 9.14 a.m.