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Multiply the diameter by (pi), or the radius by (2 pi). If you have a drawing of the circle, and the center is marked, you can measure the radius and the diameter. Just lay your ruler across the circle, and make sure that it touches the center. If the center isn't marked, then it won't be that easy.
Lay a ruler across the coin and make sure it measures from one edge through the center to the other edge. Read the diameter from the ruler.
Do lots of excercises
A million lots of them.
50 lots of 2 penny coins, 50 lots of 2 eurocents.