Olivia started jogging from the hospital to the garden at a speed of 115 m/min at 8 a.m. Her brother started jogging from the hospital later. They were beside each other at 8.43 a.m. and her brother reached the garden at 9 a.m. while Olivia was still 1700 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 8 a.m. to 9 a.m. = 1 h
From 8.43 a.m. to 9 a.m. = 17 min

Distance between the hospital and the garden
= 60 x 115 + 1700
= 6900 + 1700
= 8600 m

Distance that Olivia's brother had to cover to catch up with Olivia
= 43 x 115
= 4945 m

Distance that Olivia's brother travelled to the garden after catching up with Olivia
= 8600 - 4945
= 3655 m

Since Olivia's brother jogged 3655 m in 43 min,
Olivia's brother average speed
= 3655 ÷ 17
= 215 m/min

Duration that Olivia's brother took to jog to the garden
= 8600 ÷ 215
= 40 min

Time that Olivia's brother left the hospital:
40 min before 9 a.m. = 8.20 a.m.

Answer(s): 8.20 a.m.