Case book play

[tempestos]

Here is how I looked at it:

1. It is a running play (10-3-2).
2. The spot of enforcement ("basic spot") is where the run ends (10-4-4).
3. The end of the run occurred when B1 fumbled (2-40-9).
4. The enforcement spot becomes the goal line because the run ended in the endzone (10-5-2).

Thus, the penalty will be enforced from the goal line and result in 1st and 10 for B at the 15.

I have had a couple of very experienced white hats tell me off the top of their head that they would enforce the penalty from the 20. They considered it a touchback for B because the penalty occurred in the endzone. However, I can't find support for this interpretation in the rules.