Xuan started jogging from the salon to the garden at a speed of 100 m/min at 5 a.m. Her cousin started jogging from the salon later. They were beside each other at 5.30 a.m. and her cousin reached the garden at 6 a.m. while Xuan was still 1500 m away. If both of them travelled at a constant speed throughout the journey, what time did her cousin leave the salon?
1 h = 60 min

From 5 a.m. to 6 a.m. = 1 h
From 5.30 a.m. to 6 a.m. = 30 min

Distance between the salon and the garden
= 60 x 100 + 1500
= 6000 + 1500
= 7500 m

Distance that Xuan's cousin had to cover to catch up with Xuan
= 30 x 100
= 3000 m

Distance that Xuan's cousin travelled to the garden after catching up with Xuan
= 7500 - 3000
= 4500 m

Since Xuan's cousin jogged 4500 m in 30 min,
Xuan's cousin average speed
= 4500 ÷ 30
= 150 m/min

Duration that Xuan's cousin took to jog to the garden
= 7500 ÷ 150
= 50 min

Time that Xuan's cousin left the salon:
50 min before 6 a.m. = 5.10 a.m.

Answer(s): 5.10 a.m.