The only denominator in
S = a/(4 -r)
is (4 -r). It is also the "least common denominator."
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When solving problems with fractions, it isn't always necessary to find the "least common denominator." Often, you can work the problem easily enough just using the product of the denominators. In some cases, this may cause an extra step to reduce any fractions, but it can simplify the effort and reduce the angst.