"a pair of shoes that costs $40 was marked up by 25%. what was the markup and what is now the selling price. show your work."

Original cost of a pair of shoes = $40
Percentage by which the price of the pair of shoes is marked up = 25%
Then
Amount by which the price
of the pair of shoes is marked up = (25/100) * 40 dollars
                                                      = 40/4 dollars
                                                      = 10 dollars
So
The selling price of the shoe now = (40 + 10) dollars
                                                      = 50 dollars
So we can see from the above deduction that the price of the pair of shoes was marked up by $10 and the selling price of the pair of shoes is $50. I hope the procedure is clear enough for you to understand.    

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.