Cindy left home at 11.40 a.m. and drove towards the town. 3 hours later, her younger brother left their home and drove towards the same town at 84 km/h. When the younger brother reached the town at 7.40 p.m., Cindy had another 6 km to travel. What was Cindy’s average speed? Express your answer in mixed number.
Time that the Cindy's younger brother started driving = 2.40 p.m.
Duration that Cindy's younger brother drove from 2.40 p.m. to 7.40 p.m. = 5 h

Distance that Cindy's younger brother travelled
= 5 x 84
= 420 km

Distance that Cindy travelled by the time her younger brother reached the city
= 420 - 6
= 414 km

Time that Cindy took to travel 414 km = 8 h

Cindy's average speed
= 414 ÷ 8
= 51³₄ km/h

Answer(s): 51³₄ km/h