The usual choice would be
C. Step 1: find the price of the books; Step 2: find the price of the pens; Step 3: add the prices of the books, pens, and magazine; Step 4: add the tax.
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You can, of course, find and add the constituent prices in any order. Generally, you need to know that subtotal in order to figure the tax, which is then added to that subtotal. If you've ever done any shopping at a store where bills are added manually, or if you pay attention to any cash register receipt, you know this is the process.