A storekeeper sold a wrist watch for $660, including a sales tax of 10%. If the cost price of the watch is $500, what is the percentage profit for shopkeeper ?

$660 = Selling Price + Sales Tax (110%)
Actual selling price without including sales tax = 100 / 110 * $660 = $600
Cost Price = $500
Profit = Selling Price - Cost Price
= $600 - $500
= $100
Therefore profit percentage = Profit / Cost Price
= (100 / 500 ) * 100%
= 20%.