Answer
The dealer's cost of the calculator is $ \$ 12 $
Work Step by Step
Let $ x $ represents the dealer's cost of the calculator.
$ 25 \% $ of the dealer's cost is $ = (0.25)x $.
The selling price of a calculator is $ \$ 15 $ per night.
In mathematical form.
$ x+0.25x=15 $
$ (1+0.25)x=15 $
$ 1.25x=15 $
$ x=\frac{15}{1.25} $
$ x=12 $.
