Find surface area of a rectangle

In order to find the surface area of a rectangle:

-- Find or measure the length of the rectangle.

-- Find or measure the width of the rectangle.

-- Make sure both quantities are in the same units. 
If they're not, then convert one or the other so that they are.

-- Multiply the two numbers.

The product is the area of the rectangle, in units of
square-(the unit in which the dimensions are measured).

It would take her 20 minutes.