To solve we can use Poisson probability formula:


is the mean value of successes that happen in a given region.
x is the number of successes that are actually occurring in the given region. In our case x is 5, 4, 3, 2, 1 and 0. If a painter made 5, 4, 3, 2, 1 or no mistakes he made fewer than 6 mistakes. As these events are exclusive (you can't make 2 and 5 mistakes at the same time) we just add all those probabilities.

in our case is 4.8.
The final answer would be:
