Gem left home at 11.55 a.m. and drove towards the seaside resort. 3 hours later, her younger sister left their home and drove towards the same seaside resort at 91 km/h. When the younger sister reached the seaside resort at 7.55 p.m., Gem had another 3 km to travel. What was Gem’s average speed? Express your answer in mixed number.
Time that the Gem's younger sister started driving = 2.55 p.m.
Duration that Gem's younger sister drove from 2.55 p.m. to 7.55 p.m. = 5 h

Distance that Gem's younger sister travelled
= 5 x 91
= 455 km

Distance that Gem travelled by the time her younger sister reached the city
= 455 - 3
= 452 km

Time that Gem took to travel 452 km = 8 h

Gem's average speed
= 452 ÷ 8
= 56¹₂ km/h

Answer(s): 56¹₂ km/h