Natalie left home at 10.15 a.m. and drove towards the suburb. 3 hours later, her older sister left their home and drove towards the same suburb at 100 km/h. When the older sister reached the suburb at 6.15 p.m., Natalie had another 6 km to travel. What was Natalie’s average speed? Express your answer in mixed number.
Time that the Natalie's older sister started driving = 1.15 p.m.
Duration that Natalie's older sister drove from 1.15 p.m. to 6.15 p.m. = 5 h

Distance that Natalie's older sister travelled
= 5 x 100
= 500 km

Distance that Natalie travelled by the time her older sister reached the city
= 500 - 6
= 494 km

Time that Natalie took to travel 494 km = 8 h

Natalie's average speed
= 494 ÷ 8
= 61³₄ km/h

Answer(s): 61³₄ km/h