SOLUTION: A man made 20% by the cost of selling pocketbook at 7.50. How many pocketbook did he sell?

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