Betty left home at 11.55 a.m. and drove towards the city. 2 hours later, her auntie left their home and drove towards the same city at 80 km/h. When the auntie reached the city at 5.55 p.m., Betty had another 5 km to travel. What was Betty’s average speed? Express your answer in mixed number.
Time that the Betty's auntie started driving = 1.55 p.m.
Duration that Betty's auntie drove from 1.55 p.m. to 5.55 p.m. = 4 h

Distance that Betty's auntie travelled
= 4 x 80
= 320 km

Distance that Betty travelled by the time her auntie reached the city
= 320 - 5
= 315 km

Time that Betty took to travel 315 km = 6 h

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

Answer(s): 52¹₂ km/h