Ninja, I am not sure how well you thought this through. It is correct but maybe there was a bit of luck involved here.
"Since a 3/4 yard piece can't be cut out of a 1/3 yard-long piece, let's forget about this 1/3 yard piece"
This quote concerns me a little. There might be some left over from the 12 yard peice and together with the 1/3 yard maybe it would make another one. Of course if you looked at the left over bit afterwards this would be alright. That is probably what you were intending to do.
Otherwise it would be just
$$12\frac{1}{3}\div \frac{3}{4}$$
$${\frac{\left({\mathtt{12}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}{\left({\frac{{\mathtt{3}}}{{\mathtt{4}}}}\right)}} = {\frac{{\mathtt{148}}}{{\mathtt{9}}}} = {\mathtt{16.444\: \!444\: \!444\: \!444\: \!444\: \!4}}$$
Obviously you only want the whole bits so the answer is 16pieces