Answer
$20$
Work Step by Step
The fundamental counting principle says that if there are $|A|$ outcomes of the event $A$ and $|B|$ of the event $|B|$, then the number of outcomes of event A and event B combined is $|A|\cdot|B|$. This can easily be extended to $n$ events.
Using Fundamental Principle of Counting the number of one-topping pizzas that can be ordered is: $4\cdot5=20$
