Yoko left home at 11.35 a.m. and drove towards the town. 2 hours later, her younger brother left their home and drove towards the same town at 87 km/h. When the younger brother reached the town at 4.35 p.m., Yoko had another 6 km to travel. What was Yoko’s average speed? Express your answer in mixed number.
Time that the Yoko's younger brother started driving = 1.35 p.m.
Duration that Yoko's younger brother drove from 1.35 p.m. to 4.35 p.m. = 3 h

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

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

Time that Yoko took to travel 255 km = 5 h

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

Answer(s): 51 km/h