Pamela left home at 11.40 a.m. and drove towards the province. 2 hours later, her younger sister left their home and drove towards the same province at 87 km/h. When the younger sister reached the province at 4.40 p.m., Pamela had another 6 km to travel. What was Pamela’s average speed? Express your answer in mixed number.
Time that the Pamela's younger sister started driving = 1.40 p.m.
Duration that Pamela's younger sister drove from 1.40 p.m. to 4.40 p.m. = 3 h

Distance that Pamela's younger sister travelled
= 3 x 87
= 261 km

Distance that Pamela travelled by the time her younger sister reached the city
= 261 - 6
= 255 km

Time that Pamela took to travel 255 km = 5 h

Pamela's average speed
= 255 ÷ 5
= 51 km/h

Answer(s): 51 km/h