Sarah left home at 10.50 a.m. and drove towards the suburb. 3 hours later, her grandfather left their home and drove towards the same suburb at 99 km/h. When the grandfather reached the suburb at 5.50 p.m., Sarah had another 4 km to travel. What was Sarah’s average speed? Express your answer in mixed number.
Time that the Sarah's grandfather started driving = 1.50 p.m.
Duration that Sarah's grandfather drove from 1.50 p.m. to 5.50 p.m. = 4 h

Distance that Sarah's grandfather travelled
= 4 x 99
= 396 km

Distance that Sarah travelled by the time her grandfather reached the city
= 396 - 4
= 392 km

Time that Sarah took to travel 392 km = 7 h

Sarah's average speed
= 392 ÷ 7
= 56 km/h

Answer(s): 56 km/h