Kathy left home at 10.15 a.m. and drove towards the city. 3 hours later, her younger sister left their home and drove towards the same city at 87 km/h. When the younger sister reached the city at 6.15 p.m., Kathy had another 3 km to travel. What was Kathy’s average speed? Express your answer in mixed number.
Time that the Kathy's younger sister started driving = 1.15 p.m.
Duration that Kathy's younger sister drove from 1.15 p.m. to 6.15 p.m. = 5 h

Distance that Kathy's younger sister travelled
= 5 x 87
= 435 km

Distance that Kathy travelled by the time her younger sister reached the city
= 435 - 3
= 432 km

Time that Kathy took to travel 432 km = 8 h

Kathy's average speed
= 432 ÷ 8
= 54 km/h

Answer(s): 54 km/h