Xuan left home at 11.40 a.m. and drove towards the city. 2 hours later, her younger brother left their home and drove towards the same city at 80 km/h. When the younger brother reached the city at 5.40 p.m., Xuan had another 5 km to travel. What was Xuan’s average speed? Express your answer in mixed number.
Time that the Xuan's younger brother started driving = 1.40 p.m.
Duration that Xuan's younger brother drove from 1.40 p.m. to 5.40 p.m. = 4 h

Distance that Xuan's younger brother travelled
= 4 x 80
= 320 km

Distance that Xuan travelled by the time her younger brother reached the city
= 320 - 5
= 315 km

Time that Xuan took to travel 315 km = 6 h

Xuan's average speed
= 315 ÷ 6
= 52¹₂ km/h

Answer(s): 52¹₂ km/h