Same position means same speed

Below is a diagram representing a strobe photograph of two balls rolling (to the right) parallel to each other along two tracks. Students are asked to select where the two balls have the same speed. 

 

The red ball is travelling at a constant speed, which is seen by the equal distances between strobe flashes. The blue ball is slowing down, which is seen by the decrease in the distances it covers between strobe flashes. Since the balls have travelled the same distance between the 2nd and 3rd strobe flashes their average velocity for that interval are equal. Thus somewhere inbetween the 2nd and 3rd strobe flases the two balls would have the same speed.