If you want to turn a fraction to a decimal, you have to divide the numerator by the denominator in order to get a decimal number. If the fraction is a mixed number, you would do the same but make sure that the whole number is in front of the decimal.
If the rectangle has a fixed width of 300, it means that
where 
and 
are width and length, respetively.
So given the length , the are would be
Divide 27 and 3. then divide 31 and 4. you get 9 and 7.75. subtract those. you get 1.25
