They might even ask you to stop playing blackjack there.
You bet $10. You're dealt an 8 and a 3, so you have a total of 11. You double down, putting $10 more into action. You're dealt a card worth 10, so you have a total of 21 with $20 in action.
That's why, if you have a hard total of 16, if the dealer is showing a 7, you should take a hit. Even though you're likely to go bust, it's more likely that the dealer has a hand total of 17 or better, and he'll beat you if you stand on that 16.
Card counter have simple and complicated ways of tracking this ratio. Most of them use a simple count called the "hi lo" count. It works like this: