Xandra started jogging from the museum to the park at a speed of 115 m/min at 7 a.m. Her sister started jogging from the museum later. They were beside each other at 7.40 a.m. and her sister reached the park at 8 a.m. while Xandra was still 1700 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the museum?
1 h = 60 min

From 7 a.m. to 8 a.m. = 1 h
From 7.40 a.m. to 8 a.m. = 20 min

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

Distance that Xandra's sister had to cover to catch up with Xandra
= 40 x 115
= 4600 m

Distance that Xandra's sister travelled to the park after catching up with Xandra
= 8600 - 4600
= 4000 m

Since Xandra's sister jogged 4000 m in 40 min,
Xandra's sister average speed
= 4000 ÷ 20
= 200 m/min

Duration that Xandra's sister took to jog to the park
= 8600 ÷ 200
= 43 min

Time that Xandra's sister left the museum:
43 min before 8 a.m. = 7.17 a.m.

Answer(s): 7.17 a.m.