Diana left home at 11.40 a.m. and drove towards the seaside resort. 3 hours later, her older sister left their home and drove towards the same seaside resort at 90 km/h. When the older sister reached the seaside resort at 7.40 p.m., Diana had another 2 km to travel. What was Diana’s average speed? Express your answer in mixed number.
Time that the Diana's older sister started driving = 2.40 p.m.
Duration that Diana's older sister drove from 2.40 p.m. to 7.40 p.m. = 5 h

Distance that Diana's older sister travelled
= 5 x 90
= 450 km

Distance that Diana travelled by the time her older sister reached the city
= 450 - 2
= 448 km

Time that Diana took to travel 448 km = 8 h

Diana's average speed
= 448 ÷ 8
= 56 km/h

Answer(s): 56 km/h