Hey there!
We can set this up a proportion to figure out the percent we need.
[tex]\frac{mins}{percent} = \frac{15}{5} =\frac{60}{p}[/tex]
If you can see, the second numerator is four times the first. So, we multiply the bottom by four and we get 20.
Therefore, the probability if observing at least one mistake made by this professor is 20%.
I hope this helps!