Irene left home at 11.10 a.m. and drove towards the suburb. 2 hours later, her mother left their home and drove towards the same suburb at 99 km/h. When the mother reached the suburb at 5.10 p.m., Irene had another 6 km to travel. What was Irene’s average speed? Express your answer in mixed number.
Time that the Irene's mother started driving = 1.10 p.m.
Duration that Irene's mother drove from 1.10 p.m. to 5.10 p.m. = 4 h

Distance that Irene's mother travelled
= 4 x 99
= 396 km

Distance that Irene travelled by the time her mother reached the city
= 396 - 6
= 390 km

Time that Irene took to travel 390 km = 6 h

Irene's average speed
= 390 ÷ 6
= 65 km/h

Answer(s): 65 km/h