How many different 4-digit personal identification numbers (PINs) can be made from the digit 0 through 9 if no digits repeat?
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<em>Comment on simplifying rational expressions</em>
Division of fractions works the same whether you're working with numbers or polynomials (or anything else). Dividing by something is the same as multiplying by its inverse (reciprocal).
(a/b)/(c/d) = (a/b)·(d/c)
I learned this as "invert and multiply". I've recently seen it referred to as "copy dot flip", meaning you copy the numerator, use a dot symbol to indicate multipication, then flip the denominator (make its reciprocal) to become what you're multiplying by.