Min left home at 11.20 a.m. and drove towards the countryside. 3 hours later, her younger sister left their home and drove towards the same countryside at 100 km/h. When the younger sister reached the countryside at 7.20 p.m., Min had another 4 km to travel. What was Min’s average speed? Express your answer in mixed number.
Time that the Min's younger sister started driving = 2.20 p.m.
Duration that Min's younger sister drove from 2.20 p.m. to 7.20 p.m. = 5 h

Distance that Min's younger sister travelled
= 5 x 100
= 500 km

Distance that Min travelled by the time her younger sister reached the city
= 500 - 4
= 496 km

Time that Min took to travel 496 km = 8 h

Min's average speed
= 496 ÷ 8
= 62 km/h

Answer(s): 62 km/h