Dana started jogging from the cinema to the nature reserve at a speed of 100 m/min at 9 a.m. Her sister started jogging from the cinema later. They were beside each other at 9.45 a.m. and her sister reached the nature reserve at 10 a.m. while Dana was still 1200 m away. If both of them travelled at a constant speed throughout the journey, what time did her sister leave the cinema?
1 h = 60 min

From 9 a.m. to 10 a.m. = 1 h
From 9.45 a.m. to 10 a.m. = 15 min

Distance between the cinema and the nature reserve
= 60 x 100 + 1200
= 6000 + 1200
= 7200 m

Distance that Dana's sister had to cover to catch up with Dana
= 45 x 100
= 4500 m

Distance that Dana's sister travelled to the nature reserve after catching up with Dana
= 7200 - 4500
= 2700 m

Since Dana's sister jogged 2700 m in 45 min,
Dana's sister average speed
= 2700 ÷ 15
= 180 m/min

Duration that Dana's sister took to jog to the nature reserve
= 7200 ÷ 180
= 40 min

Time that Dana's sister left the cinema:
40 min before 10 a.m. = 9.20 a.m.

Answer(s): 9.20 a.m.