Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: a tradesman sold an article at a loss of 10 % . if the selling price had been increased by $ 50 , there would have been a gain of 10 % . what was the cost price of the article ?
$ 250
$500.