Shiyun left home at 11.40 a.m. and drove towards the countryside. 2 hours later, her mother left their home and drove towards the same countryside at 89 km/h. When the mother reached the countryside at 6.40 p.m., Shiyun had another 4 km to travel. What was Shiyun’s average speed? Express your answer in mixed number.
Time that the Shiyun's mother started driving = 1.40 p.m.
Duration that Shiyun's mother drove from 1.40 p.m. to 6.40 p.m. = 5 h

Distance that Shiyun's mother travelled
= 5 x 89
= 445 km

Distance that Shiyun travelled by the time her mother reached the city
= 445 - 4
= 441 km

Time that Shiyun took to travel 441 km = 7 h

Shiyun's average speed
= 441 ÷ 7
= 63 km/h

Answer(s): 63 km/h