Word problem

[Guest]

After spending $200 dollars at the store, Eric has 20% of his money left. How much did he start with? I know the answer is $250, but I cannot figure out the equation that proves this.

[Guest]

Let X = start amount
(200/X) = (80/100)
80 x X = 200 x 100
X = 20000/80
X = 250