Illegal Participation

[Simbio]

My friend and I went back and forth on this last night, and I thought I would check with the forum members to see what your responses would be.

Situation A:

1 and 10 for A at the A40 yard line. A10 runs out of bounds on a pass play. He runs out of bounds at the B45yard line and returns at the B35 yard line. A completes a legal pass to A20 who advances to the A15 yard line. A10 was out of bounds a) prior to the pass b)during the pass c) after the pass.

The penalty is illegal participation with a 15 yard penalty, but where would you enforce it from (exact yard line.)