You may also like

problem icon

Speedy Sidney

Two trains set off at the same time from each end of a single straight railway line. A very fast bee starts off in front of the first train and flies continuously back and forth between the two trains. How far does Sidney fly before he is squashed between the two trains?

problem icon


A security camera, taking pictures each half a second, films a cyclist going by. In the film, the cyclist appears to go forward while the wheels appear to go backwards. Why?

problem icon

Walk and Ride

How far have these students walked by the time the teacher's car reaches them after their bus broke down?

Speeding Up, Slowing Down

Stage: 3 Challenge Level: Challenge Level:2 Challenge Level:2

Congratulations to those of you who managed to find the right answer.

Robyn from the Katherine Semar Junior School was one of the first to send in a correct answer:

The shape is a pentagon, the radius is 30, the dot is on the middle of the bottom left sloping edge.

Robert and Joel from Cowbridge Comprehensive also sent in correct solutions.
Here is Robert's nice piece of detective work:

I already knew that the radius of the shape was 30.

The graph had four lines before it repeated, but since one was twice as long as the others, the shape had to have five sides.

Since the speed was never zero, the dot could not have been on a corner of the shape, but in the middle of a side.

Robert goes on to consider on which side of the pentagon the dot must have been placed:

The speed of the dot would have been slowest as the side it was on came onto and off the ground.

Since the line that marked the lowest speed (which was twice as long as the others) was the last line before the graph repeated itself, the dot had to be on the side that came off the ground last.

Therefore, the shape that made that graph was a pentagon, of radius 30, with the dot in the middle of the lower left side.

Thank you Cristina, from City of London School for Girls, for sending us a very comprehensive solution