Sarah left home at 11.15 a.m. and drove towards the campsite. 2 hours later, her grandmother left their home and drove towards the same campsite at 100 km/h. When the grandmother reached the campsite at 4.15 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 = 1.15 p.m.
Duration that Sarah's grandmother drove from 1.15 p.m. to 4.15 p.m. = 3 h

Distance that Sarah's grandmother travelled
= 3 x 100
= 300 km

Distance that Sarah travelled by the time her grandmother reached the city
= 300 - 6
= 294 km

Time that Sarah took to travel 294 km = 5 h

Sarah's average speed
= 294 ÷ 5
= 58⁴₅ km/h

Answer(s): 58⁴₅ km/h