[CT1]

Originally Posted by mbyron

The correct answer has been posted in numerous formats.

1. The result of the play is a touchback.
2. B gets first choice of penalty acceptance, since they were last in possession.
3. They will accept the IF penalty, which results automatically in a double foul, and we'll replay the down.
4. If B foolishly declines the penalty for A's foul, A gets a choice and will accept the BIB penalty: 10 yards from the basic spot, which is the A20 after the touchback, so 1/10 for A from the A30.

If wouldn't be so foolish for B to accept the penalty if the play started at the B10.