Zoe started jogging from the museum to the nature reserve at a speed of 100 m/min at 9 a.m. Her brother started jogging from the museum later. They were beside each other at 9.30 a.m. and her brother reached the nature reserve at 10 a.m. while Zoe was still 1500 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the museum?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.30 a.m. to 10 a.m. = 30 min

Distance between the museum and the nature reserve
= 60 x 100 + 1500
= 6000 + 1500
= 7500 m

Distance that Zoe's brother had to cover to catch up with Zoe
= 30 x 100
= 3000 m

Distance that Zoe's brother travelled to the nature reserve after catching up with Zoe
= 7500 - 3000
= 4500 m

Since Zoe's brother jogged 4500 m in 30 min,
Zoe's brother average speed
= 4500 ÷ 30
= 150 m/min

Duration that Zoe's brother took to jog to the nature reserve
= 7500 ÷ 150
= 50 min

Time that Zoe's brother left the museum:
50 min before 10 a.m. = 9.10 a.m.

Answer(s): 9.10 a.m.