a) `text(distance)= "area of trapezium"`
b) `text(average speed)=text(distance travelled)/text(time taken)`
c) We need to find the time when the area of the trapezium is half of its original area, or `1100 text(m)^2`, as shown in the graph.
The base of this unknown trapezium has length `t`, and the top of the trapezium will have length `t-20`. So we have:
`"area of trapezium" =((a+b)h)/2`
So it will take `37.5\ "s"` to cover half the distance.