averages and percentages are rounded.

Dealer shows a 4, 5 or 6. Dealer has a 42% chance of busting. Player has a 12 with a 31% chance of busting or a 13 with a 38% chance of busting. Why is it considered the proper move to not take a card with a 12 or 13? Can someone show me the math?

Also how does one reconcile the odds of the dealers percentage to the players percentages of busting or winning? I am not sure how to do that.

Thanks.