# A tire impression left in the mud at a crime scene was 8.7 inches wide. How many centimeters is that?

The answer is $22.098$ $c m$.
$8.7 I n c h \cdot \frac{2.54 c m}{1 I n c h} = 22.098$ $c m$
We have the inches cancelling out and $c m$ remaining in the numerator, so the units are correct.
This is multiplying by 1 because we are multiplying by 2 things that are equal, that is, $2.54$ $c m = 1$ inch. You need to put the units that you want into the numerator and the units you want to cancel out in the denominator.