Face Mask Penatly -- NFHS rule
 

I do not have my rule book in front of me, but here is my question.. 1st and 10. A on the 40 yard line. A1 hands off to A2 that runs into a crowd of B players, then turns around and runs backwards and is tackled at A's 20 yard line during the tackle B3 brings A down by the face mask. My question is that a spot foul enforced from the 20 yard line for 15 yards and replay 1st down or do we go back to the previous spot and pentilze 15 form there.

The basic spot is the end of the run. Penalize 15 yards from there, no automatic 1st. 1/15 @ A35, clock on the ready.

that is what i did but started 2nd guessing my self

You may have had the NCAA or NFL rule sitting in the back of your mind.

I was at the game that might've been what caused NFL to change their enforcement spot on that. 49ers at Giants; it resulted in a new 1st & 10 (which it already was), but a yard and a half behind the previous spot.

Canadian Ruling
 

CANADIAN RULING:

We'll enforce this from the Point of Last Scrimmage. 15+AFD.

NFL:

On a running play with no change of possession, if the run ends behind the line of scrimmage, and there's a defensive penalty behind the line of scrimmage, the spot of enforcement is either from the end of the run or the spot of the foul...whichever spot is more advantageous to the offense. If enforcement leads to the offense being short of the previous spot, the ball is advanced to the previous spot along with an automatic first down.

In the OP's case, if that happened in an NFL game, the ball would be advanced to the previous spot with an automatic first down.

Well now you'll OWN it! :)