Jade started jogging from the hospital to the nature reserve at a speed of 100 m/min at 7 a.m. Her brother started jogging from the hospital later. They were beside each other at 7.45 a.m. and her brother reached the nature reserve at 8 a.m. while Jade was still 1200 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 7 a.m. to 8 a.m. = 1 h
From 7.45 a.m. to 8 a.m. = 15 min

Distance between the hospital and the nature reserve
= 60 x 100 + 1200
= 6000 + 1200
= 7200 m

Distance that Jade's brother had to cover to catch up with Jade
= 45 x 100
= 4500 m

Distance that Jade's brother travelled to the nature reserve after catching up with Jade
= 7200 - 4500
= 2700 m

Since Jade's brother jogged 2700 m in 45 min,
Jade's brother average speed
= 2700 ÷ 15
= 180 m/min

Duration that Jade's brother took to jog to the nature reserve
= 7200 ÷ 180
= 40 min

Time that Jade's brother left the hospital:
40 min before 8 a.m. = 7.20 a.m.

Answer(s): 7.20 a.m.