Explanation

To compute the average of these 15 items including tax, we add up the prices and obtain $316.80, which we will then divide by 15. You can imagine all those pre-tax prices added up with the tax included: . Whatever the prices are, the 1.1 can be factored out, so we have



Therefore,



We can do a little long division to find



$288 is the sum of the pre-tax prices, so the average is . We can perform a second long division and when we get $19 and change we can stop, since there is only one such answer. The correct answer is (B).