Erika left home at 11.30 a.m. and drove towards the campsite. 3 hours later, her auntie left their home and drove towards the same campsite at 90 km/h. When the auntie reached the campsite at 6.30 p.m., Erika had another 3 km to travel. What was Erika’s average speed? Express your answer in mixed number.
Time that the Erika's auntie started driving = 2.30 p.m.
Duration that Erika's auntie drove from 2.30 p.m. to 6.30 p.m. = 4 h

Distance that Erika's auntie travelled
= 4 x 90
= 360 km

Distance that Erika travelled by the time her auntie reached the city
= 360 - 3
= 357 km

Time that Erika took to travel 357 km = 7 h

Erika's average speed
= 357 ÷ 7
= 51 km/h

Answer(s): 51 km/h