The key to solving word (story) problems is to correctly translate the words and phrases into concise, precise math expressions (with variables and operations).
After reading and understanding the problem, assign variable(s). If possible, try to assign a variable to the value the problem asks you to find. In this problem, let x="the number"
"When a number is decreased by 20% of itself, the result is 104" means
x - 0.20x = 104
Notes: "of" usually means multiply
Percent can be written as 20% or 0.20 or 20/100 as needed
"What is the number?" means report x
O.K., the easy math:
x - 0.2x = 104
0.8x = 104 (collect terms)
x = 130 (divide both sides by 0.8)
Checking (very important):
Is 130 - 0.20(130 = 104 ?
130 - 26 = 104 ?
104 = 104 ? yes.