Gillian left home at 10.55 a.m. and drove towards the countryside. 3 hours later, her older sister left their home and drove towards the same countryside at 83 km/h. When the older sister reached the countryside at 6.55 p.m., Gillian had another 5 km to travel. What was Gillian’s average speed? Express your answer in mixed number.
Time that the Gillian's older sister started driving = 1.55 p.m.
Duration that Gillian's older sister drove from 1.55 p.m. to 6.55 p.m. = 5 h

Distance that Gillian's older sister travelled
= 5 x 83
= 415 km

Distance that Gillian travelled by the time her older sister reached the city
= 415 - 5
= 410 km

Time that Gillian took to travel 410 km = 8 h

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

Answer(s): 51¹₄ km/h