math vocab again : the product of a number and its Blank is one
If you multiply a number and its inverse, you get one. To obtain an inverse, you divide 1 by the number. 0 doesn't have an inverse, because you cannot divide by 0.
Multiply by the reciprocal. (Flip fraction) so the reciprocal of 3 is 1/3 (because 3= 3/1)