Shiyun left home at 10.30 a.m. and drove towards the campsite. 3 hours later, her younger sister left their home and drove towards the same campsite at 81 km/h. When the younger sister reached the campsite at 6.30 p.m., Shiyun had another 5 km to travel. What was Shiyun’s average speed? Express your answer in mixed number.
Time that the Shiyun's younger sister started driving = 1.30 p.m.
Duration that Shiyun's younger sister drove from 1.30 p.m. to 6.30 p.m. = 5 h

Distance that Shiyun's younger sister travelled
= 5 x 81
= 405 km

Distance that Shiyun travelled by the time her younger sister reached the city
= 405 - 5
= 400 km

Time that Shiyun took to travel 400 km = 8 h

Shiyun's average speed
= 400 ÷ 8
= 50 km/h

Answer(s): 50 km/h