So average monthly sales is y, and biannual or 6-monthly average will be 6y..
it will not depend on number of years,,
For example let y be over x years, so Total sales over x years = 12*x*y=12xy
Now bi annual means 6-monthly or 2 times in a year......So, number of bi-annual in x years = 2x
So average biannual sales = \(\frac{12xy}{2x}=6y\)
So x doesn't matter, we just require y..

(1) The period includes exactly 4 years.
so x=4, but we don't require x.

(2) y = 500.
So, answer is 6y=6*500=3000

B
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