Explanation

This is a percentage equation in one variable. Let the number be n: 20% of n added to 20 gives n, i.e. 0.2n + 20 = n. Rearranging, 0.8n = 20, so n = 25. Checking: 20% of 25 is 5, and 5 + 20 = 25, the number itself. The common error is misreading the statement as 0.2(n + 20) = n, which leads to a non-listed answer. Hence option (b).