Selina earns 8% commission on sales. on one sale, her commission was $20.40. what was the amount of that sale?

There are many different ways that one would be able to solve this problem.  I am going to show you the percent proportion, which is a/b = p/100, where p is the percentage, and a is a portion of the total number b.  So, if we substitute our known numbers into the equation, we get:

20.40/b = 8/100

To simplify this equation, we are going to use cross products, or the multiplication of the numerator of one fraction times the denominator of the other fraction, and setting these two products equal to one another.  If done correctly, you should get the simplified equation:

(100)(20.40) = (8)(b)

When we multiply the left and right sides of the equation, we get 

2040=8b

Now, we must divide both sides of the coefficient, 8, to get our final answer.

b = 255

Therefore, if Selena earns 8% commission on sales, and her commission was $20.40, then the amount of her sale was $255.00.

The answer would be a i think