a] David's speed during the period:
i] AB
speed=distance/time
time taken between AB=0.5 hours
distance=20 km
thus
speed=20/0.5=40 km/h
ii]BC
time taken between BC=1.5 hours
distance=20
speed=20/1.5=13 1/3 km/h
b] Average speed for the entire time will be:
average speed=(total distance)/(total time)
total distance=(20+20+20)=60 km
total time=3 hours
thus
average speed=60/3=20 km/h
c] David's distance 2 hours and 15 minutes from the starting point was:
distance=20+20+10=50 km