Shiyun left home at 11.50 a.m. and drove towards the countryside. 2 hours later, her younger brother left their home and drove towards the same countryside at 85 km/h. When the younger brother reached the countryside at 5.50 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 younger brother started driving = 1.50 p.m.
Duration that Shiyun's younger brother drove from 1.50 p.m. to 5.50 p.m. = 4 h

Distance that Shiyun's younger brother travelled
= 4 x 85
= 340 km

Distance that Shiyun travelled by the time her younger brother reached the city
= 340 - 4
= 336 km

Time that Shiyun took to travel 336 km = 6 h

Shiyun's average speed
= 336 ÷ 6
= 56 km/h

Answer(s): 56 km/h