NFHS - Rule 5-1-2(a)

[HLin NC]

What isn't clear? 5-1-2a in part

After a first, second or third down, a new series of downs shall be awarded only after considering the effect of any act during the down other than nonplayer or unsportsmanlike conduct fouls by A, and any dead-ball foul by B.]

Note its "comma and any dead ball foul by B" You had 2nd down, a dead ball foul by B, which occurred first in order and thus the penalty is assessed against B first. (See 10-2-5) the new series is awarded and then A's penalty is assesesed.

Mark off 15 against B to the A20, 1st down A. Mark off half the distance against A to the A10, set the chains, 1-10 A from the A-10.