problem: The sum of the first $N$ positive odd numbers is 121. What is the value of $N$?
solution: The first $N$ positive odd integers are 1, 3, $\dots$, $2N - 1$.  The sum of an arithmetic series is equal to the average of the first and last term, multiplied by the number of terms, so the sum of the first $N$ positive odd integers is \[\frac{1 + (2N - 1)}{2} \cdot N = N^2.\]If $N^2 = 121$, then $N = \boxed{11}$.