To add fractions, you find the least common multiple.
But, a faster way would be multiply the two denominators together:
x/20 + /20
How many times does 4 go into 20? 5 times. 5 times 1 equals 5.
5/20 + x/20
How many times does 5 go into 20 4 times? 4 times 1 equals 4.
5/20 + 4/20 = 9/20. This is the final answer because it can't be simplified any more.