AMC 8 2007 Problem 1

Theresa's parents have agreed to buy her tickets to see her favorite band if she spends an average of $10$ hours per week helping around the house for $6$ weeks. For the first $5$ weeks she helps around the house for $8$, $11$, $7$, $12$ and $10$ hours. How many hours must she work for the final week to earn the tickets? $\mathrm{(A)}\ 9 \mathrm{(B)}\ 10 \mathrm{(C)}\ 11 \mathrm{(D)}\ 12 \mathrm{(E)}\ 13$