Roshel left home at 11.10 a.m. and drove towards the countryside. 2 hours later, her younger brother left their home and drove towards the same countryside at 79 km/h. When the younger brother reached the countryside at 5.10 p.m., Roshel had another 4 km to travel. What was Roshel’s average speed? Express your answer in mixed number.
Time that the Roshel's younger brother started driving = 1.10 p.m.
Duration that Roshel's younger brother drove from 1.10 p.m. to 5.10 p.m. = 4 h

Distance that Roshel's younger brother travelled
= 4 x 79
= 316 km

Distance that Roshel travelled by the time her younger brother reached the city
= 316 - 4
= 312 km

Time that Roshel took to travel 312 km = 6 h

Roshel's average speed
= 312 ÷ 6
= 52 km/h

Answer(s): 52 km/h