Risa left home at 11.50 a.m. and drove towards the countryside. 3 hours later, her grandmother left their home and drove towards the same countryside at 83 km/h. When the grandmother reached the countryside at 7.50 p.m., Risa had another 3 km to travel. What was Risa’s average speed? Express your answer in mixed number.
Time that the Risa's grandmother started driving = 2.50 p.m.
Duration that Risa's grandmother drove from 2.50 p.m. to 7.50 p.m. = 5 h

Distance that Risa's grandmother travelled
= 5 x 83
= 415 km

Distance that Risa travelled by the time her grandmother reached the city
= 415 - 3
= 412 km

Time that Risa took to travel 412 km = 8 h

Risa's average speed
= 412 ÷ 8
= 51¹₂ km/h

Answer(s): 51¹₂ km/h