Xuan left home at 11.50 a.m. and drove towards the town. 2 hours later, her older sister left their home and drove towards the same town at 87 km/h. When the older sister reached the town at 5.50 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 = 1.50 p.m.
Duration that Xuan's older sister drove from 1.50 p.m. to 5.50 p.m. = 4 h

Distance that Xuan's older sister travelled
= 4 x 87
= 348 km

Distance that Xuan travelled by the time her older sister reached the city
= 348 - 6
= 342 km

Time that Xuan took to travel 342 km = 6 h

Xuan's average speed
= 342 ÷ 6
= 57 km/h

Answer(s): 57 km/h