Let N be the number of items sold and p the price.
Since the variation is inverse, then the relation between N and p is:

For N=20000 and p = $9.5, we get the formula:

If p = 8.75, then the number of items sold can be computed using the formula:
-1-4x+7x=1x+4x
-1-4x+7x=1x+4x
-1+-4x+7x=x+4x
(-4x+7x)+(-1)=(x+4x)
3x+-1=5x
3x-1=5x
3x-1-5x=5x-5x
-2x-1=0
-2x-1+1=0+1
-2x=1
-2x/-2=1/2
x=-1/2
The number of guitar lessons that can be completed in 7 months is 2(7) = 14.