If a store uses a selling price-based markup of 40% and an item costs the store $300, what selling price would the store set for the item?

Selling Price is $500

Mark up on selling price has this formula:

Markup Amount / Selling Price = Markup Percentage.

OR

Item Cost / (1.00 - Mark up Percentage) = Price.

Since the problem gave us the item cost and markup percentage, we use the 2nd formula:
Item cost = 300
selling price-based markup = 40%

300 / (1-0.40) = 300 / 0.60 = 500 Selling Price.

Selling Price :      500
Item cost      :   -  300
Mark up amt :      200

With the above figures, we can compute for the mark up percentage using the 1st formula.

Mark up amount / selling price = mark up percentage

200 / 500 = 0.40 or 40%