need help with a formula!!

this is what i need to do:

insert width and length of a rectangular.

how many squares of 20 cms side can u put inside that rectangular.

and whats the area left with no squares.

with the width and lenght of the rectangular i can calculate the area of the rectangle.

Using the area i can calculate how many 20x20 is inside that area. but if its a 30x35 rectangle A=950, and 20x20 is 400. it says it can hold 2 squares ( 400x 2 =800 < 950) , but its not true it can only hold 1 and some area is left over.

How do i have to write to be sure that it reads the square as a 20x20 and not just an area of 400?