AoPS Wiki talk:Problem of the Day/June 26, 2011

Problem

AoPSWiki:Problem of the Day/June 26, 2011

Solutions

Let $x$ equal the cost of the calculator before sales tax (the "base price"). The amount of sales tax can be represented as $0.07x$ since it is given as $7$ percent. We know that the final price of the calculator, which is equal to the sum of the base price of the calculator and the amount of sales tax, is $13.91$. Thus, $x + 0.07x = 13.91$, or $1.07x=13.91$. Dividing both sides by $1.07$, we get $x=\boxed{13.00}$.