Min started jogging from the library to the garden at a speed of 100 m/min at 9 a.m. Her brother started jogging from the library later. They were beside each other at 9.30 a.m. and her brother reached the garden at 10 a.m. while Min was still 1500 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the library?
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 library and the garden
= 60 x 100 + 1500
= 6000 + 1500
= 7500 m

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

Distance that Min's brother travelled to the garden after catching up with Min
= 7500 - 3000
= 4500 m

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

Duration that Min's brother took to jog to the garden
= 7500 ÷ 150
= 50 min

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

Answer(s): 9.10 a.m.