Anna started jogging from the school to the nature reserve at a speed of 95 m/min at 8 a.m. Her brother started jogging from the school later. They were beside each other at 8.40 a.m. and her brother reached the nature reserve at 9 a.m. while Anna was still 1900 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the school?
1 h = 60 min

From 8 a.m. to 9 a.m. = 1 h
From 8.40 a.m. to 9 a.m. = 20 min

Distance between the school and the nature reserve
= 60 x 95 + 1900
= 5700 + 1900
= 7600 m

Distance that Anna's brother had to cover to catch up with Anna
= 40 x 95
= 3800 m

Distance that Anna's brother travelled to the nature reserve after catching up with Anna
= 7600 - 3800
= 3800 m

Since Anna's brother jogged 3800 m in 40 min,
Anna's brother average speed
= 3800 ÷ 20
= 190 m/min

Duration that Anna's brother took to jog to the nature reserve
= 7600 ÷ 190
= 40 min

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

Answer(s): 8.20 a.m.