The expression that gives the best estimate of the number of cotton balls is "total cotton balls = (number of bags) * (cotton balls per bag)" and the total cotton balls that the teacher bought is 612.
Step-by-step explanation:
Since all the bags have the same amount of cotton balls we can find the total number of cotton balls by adding the amount of each bag nine times, that would be the same as multiplying the number of bags by the number of cotton balls on each bag. Therefore the expression that gives us the total number of cotton balls is:
total cotton balls = (number of bags) * (cotton balls per bag)
