For part b, the question asks for a sketch. Actually, to be more precise, we have to draw the graph on graph paper.

Part c relies on values taken from graph paper. My 27 s was just a 'random estimate based on eye power'.

For part (d), we notice that doubling the distance does not quite double the time taken. Take for example, the fastest runner who runs 100 m in 12 s will run 200 m in 25 s. We know that 12 x 2 = 24, and 1 sec difference is counted as a large margin when it comes to sports (for athletes, every 0.1 s already counts).

For part (e), it makes sense that a runner's endurance and energy level drops as the run progresses, thus the runner is expected to slow down eventually (in theory only; in practice, I know runners would sprint at the last minute once they know.that their run is about to finish).