You find a common denominator first. Multiply 3/5 by 4, making the fraction 12/20. Multiply 1/4 by 5, making the fraction 5/20. Now, both fractions have a denominator of 20. You now add the numerator, not the denominator. The sum of 12/20 and 5/20 is 17/20. The fraction cannot be further simplified. 17 and 20 do not share a common factor other than 1. If you need it in a decimal form, it is 0.85.
