Cathy left home at 11.50 a.m. and drove towards the countryside. 3 hours later, her younger brother left their home and drove towards the same countryside at 86 km/h. When the younger brother reached the countryside at 7.50 p.m., Cathy had another 2 km to travel. What was Cathy’s average speed? Express your answer in mixed number.
Time that the Cathy's younger brother started driving = 2.50 p.m.
Duration that Cathy's younger brother drove from 2.50 p.m. to 7.50 p.m. = 5 h

Distance that Cathy's younger brother travelled
= 5 x 86
= 430 km

Distance that Cathy travelled by the time her younger brother reached the city
= 430 - 2
= 428 km

Time that Cathy took to travel 428 km = 8 h

Cathy's average speed
= 428 ÷ 8
= 53¹₂ km/h

Answer(s): 53¹₂ km/h