Answer:
D. 25% decrease
Explanation:
When we have multiple choice questions, generally the best first step is to narrow down your options. We can see that our options are either increase or decrease. Obviously, if the class read 40 books in June and then 30 in July, that's a decrease. So option C is out of the question.
We still have three options left, so we have to now figure out the exact percentage decrease. Let's find out what percentage of 40 is 30.
(Remember that percentages are parts of 1. 100%=1, 50%=.5)
So 30 is 75% of 40. And what percentage did we go down from 100%? 25%.
There was a 25% decrease in the number of books read.
I hope this helps!