Barbara left home at 11.40 a.m. and drove towards the city. 2 hours later, her younger brother left their home and drove towards the same city at 76 km/h. When the younger brother reached the city at 5.40 p.m., Barbara had another 4 km to travel. What was Barbara’s average speed? Express your answer in mixed number.
Time that the Barbara's younger brother started driving = 1.40 p.m.
Duration that Barbara's younger brother drove from 1.40 p.m. to 5.40 p.m. = 4 h

Distance that Barbara's younger brother travelled
= 4 x 76
= 304 km

Distance that Barbara travelled by the time her younger brother reached the city
= 304 - 4
= 300 km

Time that Barbara took to travel 300 km = 6 h

Barbara's average speed
= 300 ÷ 6
= 50 km/h

Answer(s): 50 km/h