Rachel left home at 11.40 a.m. and drove towards the suburb. 3 hours later, her older sister left their home and drove towards the same suburb at 86 km/h. When the older sister reached the suburb at 7.40 p.m., Rachel had another 6 km to travel. What was Rachel’s average speed? Express your answer in mixed number.
Time that the Rachel's older sister started driving = 2.40 p.m.
Duration that Rachel's older sister drove from 2.40 p.m. to 7.40 p.m. = 5 h

Distance that Rachel's older sister travelled
= 5 x 86
= 430 km

Distance that Rachel travelled by the time her older sister reached the city
= 430 - 6
= 424 km

Time that Rachel took to travel 424 km = 8 h

Rachel's average speed
= 424 ÷ 8
= 53 km/h

Answer(s): 53 km/h