Erika started jogging from the mall to the park at a speed of 115 m/min at 7 a.m. Her brother started jogging from the mall later. They were beside each other at 7.43 a.m. and her brother reached the park at 8 a.m. while Erika was still 1700 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the mall?
1 h = 60 min

From 7 a.m. to 8 a.m. = 1 h
From 7.43 a.m. to 8 a.m. = 17 min

Distance between the mall and the park
= 60 x 115 + 1700
= 6900 + 1700
= 8600 m

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

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

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

Duration that Erika's brother took to jog to the park
= 8600 ÷ 215
= 40 min

Time that Erika's brother left the mall:
40 min before 8 a.m. = 7.20 a.m.

Answer(s): 7.20 a.m.