Risa left home at 11.15 a.m. and drove towards the city. 3 hours later, her older brother left their home and drove towards the same city at 82 km/h. When the older brother reached the city at 7.15 p.m., Risa had another 4 km to travel. What was Risa’s average speed? Express your answer in mixed number.
Time that the Risa's older brother started driving = 2.15 p.m.
Duration that Risa's older brother drove from 2.15 p.m. to 7.15 p.m. = 5 h

Distance that Risa's older brother travelled
= 5 x 82
= 410 km

Distance that Risa travelled by the time her older brother reached the city
= 410 - 4
= 406 km

Time that Risa took to travel 406 km = 8 h

Risa's average speed
= 406 ÷ 8
= 50³₄ km/h

Answer(s): 50³₄ km/h