METHOD 1: 

Do the calculation with decimals.

Then convert the answer into a fraction.

0.31 - ¼ =

0.31 - 0.25 =

0.06 = 6/100 = 3/50

METHOD 2:

Do the calculation with fractions.

0.31 - ¼ = 

31/100 - ¼ =

31/100 - 25/100 = 

6/100 = 3/50

As you can see, either way the result is the same. If you dislike finding the common denominator, then calculating with decimals might be easier. But if one of the numbers is a repeating decimal, then calculating with fractions might be easier. As a good problem solver, you should always pick the method that feels like less work for you.