Answer
R cannot be equal to zero because once the operation is changed to multiplication after inverting the divisor, R becomes a denominator.
Work Step by Step
RECALL:
$\\\dfrac{P}{Q} \div \dfrac{R}{S} = \dfrac{P}{Q} \cdot \dfrac{S}{R}$
Notice that once the operation is changed to multiplication after inverting the divisor, $R$ goes to the denominator.
A denominator cannot be zero therefore $R$ cannot be equal to zero.
