Tammy left home at 11.55 a.m. and drove towards the suburb. 3 hours later, her younger brother left their home and drove towards the same suburb at 88 km/h. When the younger brother reached the suburb at 7.55 p.m., Tammy had another 4 km to travel. What was Tammy’s average speed? Express your answer in mixed number.
Time that the Tammy's younger brother started driving = 2.55 p.m.
Duration that Tammy's younger brother drove from 2.55 p.m. to 7.55 p.m. = 5 h

Distance that Tammy's younger brother travelled
= 5 x 88
= 440 km

Distance that Tammy travelled by the time her younger brother reached the city
= 440 - 4
= 436 km

Time that Tammy took to travel 436 km = 8 h

Tammy's average speed
= 436 ÷ 8
= 54¹₂ km/h

Answer(s): 54¹₂ km/h