Xuan left home at 11.20 a.m. and drove towards the suburb. 2 hours later, her older sister left their home and drove towards the same suburb at 81 km/h. When the older sister reached the suburb at 6.20 p.m., Xuan had another 6 km to travel. What was Xuan’s average speed? Express your answer in mixed number.
Time that the Xuan's older sister started driving = 1.20 p.m.
Duration that Xuan's older sister drove from 1.20 p.m. to 6.20 p.m. = 5 h

Distance that Xuan's older sister travelled
= 5 x 81
= 405 km

Distance that Xuan travelled by the time her older sister reached the city
= 405 - 6
= 399 km

Time that Xuan took to travel 399 km = 7 h

Xuan's average speed
= 399 ÷ 7
= 57 km/h

Answer(s): 57 km/h