Gabby left home at 10.15 a.m. and drove towards the suburb. 3 hours later, her older sister left their home and drove towards the same suburb at 99 km/h. When the older sister reached the suburb at 6.15 p.m., Gabby had another 5 km to travel. What was Gabby’s average speed? Express your answer in mixed number.
Time that the Gabby's older sister started driving = 1.15 p.m.
Duration that Gabby's older sister drove from 1.15 p.m. to 6.15 p.m. = 5 h

Distance that Gabby's older sister travelled
= 5 x 99
= 495 km

Distance that Gabby travelled by the time her older sister reached the city
= 495 - 5
= 490 km

Time that Gabby took to travel 490 km = 8 h

Gabby's average speed
= 490 ÷ 8
= 61¹₄ km/h

Answer(s): 61¹₄ km/h