Vanessa left home at 11.55 a.m. and drove towards the city. 2 hours later, her younger brother left their home and drove towards the same city at 87 km/h. When the younger brother reached the city at 4.55 p.m., Vanessa had another 5 km to travel. What was Vanessa’s average speed? Express your answer in mixed number.
Time that the Vanessa's younger brother started driving = 1.55 p.m.
Duration that Vanessa's younger brother drove from 1.55 p.m. to 4.55 p.m. = 3 h

Distance that Vanessa's younger brother travelled
= 3 x 87
= 261 km

Distance that Vanessa travelled by the time her younger brother reached the city
= 261 - 5
= 256 km

Time that Vanessa took to travel 256 km = 5 h

Vanessa's average speed
= 256 ÷ 5
= 51¹₅ km/h

Answer(s): 51¹₅ km/h