Pioneer to Holland Village
Speed : 20km per hour
Time Taken : 39min = 39min/60min = 0.65 hour
Distance travelled = speed x time = 20x0.65= 13km

Holland Village to Clarke Quay
Distance: 7.5km
Time taken : 0.5 hour

Therefore, from Pioneer to Clarke Quay
Total Distance travelled = 13+7.5 = 20.5km
Total Time taken = 0.65 + 0.5 = 1.15 hr

Average speed = (total distance)/(total time)
= 20.5/11.5 = 17.8

Hope it helps! :)