That's not enough information. 

All we can find from the information given here
is the cost of each pocketbook.

Let x = the cost of 1 pocketbook

Then his profit is 20% of x, or 0.2x

So  COST + PROFIT = SELLING PRICE

         x + 0.2x = 7.50
             1.2x = 7.50
                x = 
                x = 6.25

So each pocketbook cost him $6.25 and he sold each pocketbook
for $7.50, making a profit of $1.25 on each pocketbook.  And
$1.25 is 20% of $6.25.  

So he might have sold only 1 pocketbook or a million pocketbooks.
His profit would be 20% regardless of how many pocketbooks he sold.  

If you'll type the ENTIRE problem, including ALL the information
in the thank-you note form, I will help you.

Edwin