Tina left home at 11.30 a.m. and drove towards the countryside. 2 hours later, her older sister left their home and drove towards the same countryside at 86 km/h. When the older sister reached the countryside at 4.30 p.m., Tina had another 3 km to travel. What was Tina’s average speed? Express your answer in mixed number.
Time that the Tina's older sister started driving = 1.30 p.m.
Duration that Tina's older sister drove from 1.30 p.m. to 4.30 p.m. = 3 h

Distance that Tina's older sister travelled
= 3 x 86
= 258 km

Distance that Tina travelled by the time her older sister reached the city
= 258 - 3
= 255 km

Time that Tina took to travel 255 km = 5 h

Tina's average speed
= 255 ÷ 5
= 51 km/h

Answer(s): 51 km/h