Sarah left home at 11.50 a.m. and drove towards the town. 3 hours later, her grandmother left their home and drove towards the same town at 89 km/h. When the grandmother reached the town at 6.50 p.m., Sarah had another 6 km to travel. What was Sarah’s average speed? Express your answer in mixed number.
Time that the Sarah's grandmother started driving = 2.50 p.m.
Duration that Sarah's grandmother drove from 2.50 p.m. to 6.50 p.m. = 4 h

Distance that Sarah's grandmother travelled
= 4 x 89
= 356 km

Distance that Sarah travelled by the time her grandmother reached the city
= 356 - 6
= 350 km

Time that Sarah took to travel 350 km = 7 h

Sarah's average speed
= 350 ÷ 7
= 50 km/h

Answer(s): 50 km/h