The product of a number and 12 is less than 5

First we need to have same units before comparing them. That is, either both of them are in cm or both of them are in mm. So if we need to convert 12.5 cm to mm, we know that 1 cm =10 mm. So we have to find out how many mm are in 12.5 cm. And let 12.5 cm =x mm. So to find the value of x , we set a proportion and solve for x. That is

The units cancel out and we do cross multiplication. That is

x=125

So 12.5 cm =125 mm. Therefore the correct proportion is C.