The answer is b, f[c(p)]=0.9265p.
f[c(p)]=f(0.85p)=1.09(0.85p)=0.9265p
The answer is not a whole number, so I'm not sure if I am 100% right.
The answer I got was 10.66666667 Pieces. Of course you wouldn't type that many 6's, just as many as it asks for. lol