Xuan started jogging from the salon to the garden at a speed of 105 m/min at 6 a.m. Her brother started jogging from the salon later. They were beside each other at 6.41 a.m. and her brother reached the garden at 7 a.m. while Xuan was still 1900 m away. If both of them travelled at a constant speed throughout the journey, what time did her brother leave the salon?
1 h = 60 min

From 6 a.m. to 7 a.m. = 1 h
From 6.41 a.m. to 7 a.m. = 19 min

Distance between the salon and the garden
= 60 x 105 + 1900
= 6300 + 1900
= 8200 m

Distance that Xuan's brother had to cover to catch up with Xuan
= 41 x 105
= 4305 m

Distance that Xuan's brother travelled to the garden after catching up with Xuan
= 8200 - 4305
= 3895 m

Since Xuan's brother jogged 3895 m in 41 min,
Xuan's brother average speed
= 3895 ÷ 19
= 205 m/min

Duration that Xuan's brother took to jog to the garden
= 8200 ÷ 205
= 40 min

Time that Xuan's brother left the salon:
40 min before 7 a.m. = 6.20 a.m.

Answer(s): 6.20 a.m.