Answer:
The usual size of the container is 15 ounces.
Step-by-step explanation:
Consider the provided information.
A protein mix claims to contain a bonus of 20% more mix than the usual size container.
Let the usual size of the container was x ounces.
They increase 20% of usual size and the bonus size is 18 ounces.
Therefore, the new size should be:

(∴20%=
=0.20)



Hence, the usual size of the container is 15 ounces.