Yen left home at 11.55 a.m. and drove towards the city. 3 hours later, her younger brother left their home and drove towards the same city at 90 km/h. When the younger brother reached the city at 7.55 p.m., Yen had another 4 km to travel. What was Yen’s average speed? Express your answer in mixed number.
Time that the Yen's younger brother started driving = 2.55 p.m.
Duration that Yen's younger brother drove from 2.55 p.m. to 7.55 p.m. = 5 h

Distance that Yen's younger brother travelled
= 5 x 90
= 450 km

Distance that Yen travelled by the time her younger brother reached the city
= 450 - 4
= 446 km

Time that Yen took to travel 446 km = 8 h

Yen's average speed
= 446 ÷ 8
= 55³₄ km/h

Answer(s): 55³₄ km/h