One fourth of a number is 1 more than one fifth of the same number

The most important part of this question is understanding the words of the problem. The trick lies in the words of the problem. Once the web of words is simplified, the problem becomes very easy. The main thing is converting the words into numbers.
Let us assume the unknown number = x
Then from the question we get
(x/4) = (x/5) + 1
(x/4) - (x/5) = 1
(5x - 4x)/20 = 1
x/20 = 1
x = 20
So the unknown number is 20