Irene started jogging from the farm to the garden at a speed of 100 m/min at 6 a.m. Her brother started jogging from the farm later. They were beside each other at 6.42 a.m. and her brother reached the garden at 7 a.m. while Irene was still 1800 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the farm?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.42 a.m. to 7 a.m. = 18 min

Distance between the farm and the garden
= 60 x 100 + 1800
= 6000 + 1800
= 7800 m

Distance that Irene's brother had to cover to catch up with Irene
= 42 x 100
= 4200 m

Distance that Irene's brother travelled to the garden after catching up with Irene
= 7800 - 4200
= 3600 m

Since Irene's brother jogged 3600 m in 42 min,
Irene's brother average speed
= 3600 ÷ 18
= 200 m/min

Duration that Irene's brother took to jog to the garden
= 7800 ÷ 200
= 39 min

Time that Irene's brother left the farm:
39 min before 7 a.m. = 6.21 a.m.

Answer(s): 6.21 a.m.