Ivory left home at 11.45 a.m. and drove towards the town. 2 hours later, her older sister left their home and drove towards the same town at 90 km/h. When the older sister reached the town at 4.45 p.m., Ivory had another 4 km to travel. What was Ivory’s average speed? Express your answer in mixed number.
Time that the Ivory's older sister started driving = 1.45 p.m.
Duration that Ivory's older sister drove from 1.45 p.m. to 4.45 p.m. = 3 h

Distance that Ivory's older sister travelled
= 3 x 90
= 270 km

Distance that Ivory travelled by the time her older sister reached the city
= 270 - 4
= 266 km

Time that Ivory took to travel 266 km = 5 h

Ivory's average speed
= 266 ÷ 5
= 53¹₅ km/h

Answer(s): 53¹₅ km/h