In order to do this sum (without using a calculator) you must find the lowest common multiple of 5 and 4. It is 20. This means that the bottom part of both fractions must become 20. /20 + /20. For the top half, you must multiply the original top halves by what you multiplied the bottom halves. 1 X 4 = 4. 3 X 5 = 15. The question now becomes: 4/20 + 15/ 20. The answer is 19/ 20