The front wheel makes 60 rotations per minute. How many rotations will the back wheel make in 1 hour?  

The front wheel of a k-nex car is 6.5 cm. in diameter. The first thing you do is get how far the first wheel goes in 1 minute by finding the circumference and multiplying it by the rotations per minute.

6.5cm x Pi = 20.42cm (circumference)
20.42cm x 60 rotations = 1225.2cm in 1 minute for the front wheel.

If the front wheel travels 1225.2cm then the back wheel travels the same distance.

The back wheel has a 9 cm diameter.  Now you find the circumference of the back wheel.

9 x Pi = 28.27cm (circumference of the back wheel)
Now 1225.2cm / 28.27cm = the rotations of the back wheel in 1 minute.
You get 43.33 rotations.

Now since its 60 minute you do
43.33 rotations x 60 minutes = the rotations per hour which is about 2600 rotations.