Lucy left home at 11.50 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 7.50 p.m., Lucy had another 3 km to travel. What was Lucy’s average speed? Express your answer in mixed number.
Time that the Lucy's younger sister started driving = 2.50 p.m.
Duration that Lucy's younger sister drove from 2.50 p.m. to 7.50 p.m. = 5 h

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

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

Time that Lucy took to travel 432 km = 8 h

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

Answer(s): 54 km/h