Will you please work #20 in section 4, Test 1.

Sure! When you see percents, you should almost always think about plugging in 100. In this case, that works like a charm. Say the laptop originally cost $100. When it was put on sale for 20% off, the price went to $80. Buying an $80 laptop with 8% sales tax will cost . Which answer choice gives you 100 when you plug in 86.4 for *p*? Only choice D.

If you want to know WHY that works, then look at the algebra. Forget about the tax for a moment. Alma bought the computer at a 20% discount. So if the computer originally cost *x* dollars, then the price she paid can be calculated thusly:

Now we need to add 8% sales tax to that. We do that by taking the price she paid and increasing it by 8%:

That expression gives you the price Alma paid for the laptop—which the question calls *p*. So we can say:

Since the question asks you for the original price, *x*, in terms of *p*, solve for *x *by dividing.