We cannot answer either of those questions without knowing how many pieces are left.
If there are x pieces...
a. 65x, since each piece adds 65 centimeters to the total length.
b. 65(x-1). Same logic as above, except we subtract one from x since we have one less piece of yarn.
Also, make sure you place your question in the right category! You put this question in English, but it looks to me like more of a Math question.
