Kathy left home at 10.35 a.m. and drove towards the city. 3 hours later, her grandfather left their home and drove towards the same city at 81 km/h. When the grandfather reached the city at 6.35 p.m., Kathy had another 5 km to travel. What was Kathy’s average speed? Express your answer in mixed number.
Time that the Kathy's grandfather started driving = 1.35 p.m.
Duration that Kathy's grandfather drove from 1.35 p.m. to 6.35 p.m. = 5 h

Distance that Kathy's grandfather travelled
= 5 x 81
= 405 km

Distance that Kathy travelled by the time her grandfather reached the city
= 405 - 5
= 400 km

Time that Kathy took to travel 400 km = 8 h

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

Answer(s): 50 km/h