Mary started jogging from the playground to the garden at a speed of 96 m/min at 6 a.m. Her brother started jogging from the playground later. They were beside each other at 6.35 a.m. and her brother reached the garden at 7 a.m. while Mary was still 1600 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the playground?
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 playground and the garden
= 60 x 96 + 1600
= 5760 + 1600
= 7360 m

Distance that Mary's brother had to cover to catch up with Mary
= 35 x 96
= 3360 m

Distance that Mary's brother travelled to the garden after catching up with Mary
= 7360 - 3360
= 4000 m

Since Mary's brother jogged 4000 m in 35 min,
Mary's brother average speed
= 4000 ÷ 25
= 160 m/min

Duration that Mary's brother took to jog to the garden
= 7360 ÷ 160
= 46 min

Time that Mary's brother left the playground:
46 min before 7 a.m. = 6.14 a.m.

Answer(s): 6.14 a.m.