Zara started jogging from the hospital to the reservoir at a speed of 115 m/min at 6 a.m. Her brother started jogging from the hospital later. They were beside each other at 6.35 a.m. and her brother reached the reservoir at 7 a.m. while Zara was still 1500 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the hospital?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.35 a.m. to 7 a.m. = 25 min

Distance between the hospital and the reservoir
= 60 x 115 + 1500
= 6900 + 1500
= 8400 m

Distance that Zara's brother had to cover to catch up with Zara
= 35 x 115
= 4025 m

Distance that Zara's brother travelled to the reservoir after catching up with Zara
= 8400 - 4025
= 4375 m

Since Zara's brother jogged 4375 m in 35 min,
Zara's brother average speed
= 4375 ÷ 25
= 175 m/min

Duration that Zara's brother took to jog to the reservoir
= 8400 ÷ 175
= 48 min

Time that Zara's brother left the hospital:
48 min before 7 a.m. = 6.12 a.m.

Answer(s): 6.12 a.m.