Diana left home at 11.40 a.m. and drove towards the seaside resort. 3 hours later, her grandmother left their home and drove towards the same seaside resort at 83 km/h. When the grandmother reached the seaside resort at 7.40 p.m., Diana had another 5 km to travel. What was Diana’s average speed? Express your answer in mixed number.
Time that the Diana's grandmother started driving = 2.40 p.m.
Duration that Diana's grandmother drove from 2.40 p.m. to 7.40 p.m. = 5 h

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

Distance that Diana travelled by the time her grandmother reached the city
= 415 - 5
= 410 km

Time that Diana took to travel 410 km = 8 h

Diana's average speed
= 410 ÷ 8
= 51¹₄ km/h

Answer(s): 51¹₄ km/h