Answer:
7000 cubic yards
Explanation:
Given :The volume of water in a reservoir decreases 21,000 cubic yards over a 3-month period.
To Find:What is the average change in volume per month?
Solution:
Since we are given that The volumes of water decreases by amount in 3 months = 21000 cubic yards
So, the volume decreased per month =

=

Thus the volume decreased per month is 7000 cubic yards .
Hence the average change in volume per month is 7000 cubic yards