Jean left home at 11.55 a.m. and drove towards the seaside resort. 3 hours later, her grandfather left their home and drove towards the same seaside resort at 92 km/h. When the grandfather reached the seaside resort at 7.55 p.m., Jean had another 4 km to travel. What was Jean’s average speed? Express your answer in mixed number.
Time that the Jean's grandfather started driving = 2.55 p.m.
Duration that Jean's grandfather drove from 2.55 p.m. to 7.55 p.m. = 5 h

Distance that Jean's grandfather travelled
= 5 x 92
= 460 km

Distance that Jean travelled by the time her grandfather reached the city
= 460 - 4
= 456 km

Time that Jean took to travel 456 km = 8 h

Jean's average speed
= 456 ÷ 8
= 57 km/h

Answer(s): 57 km/h