The first term of an Arithmetic Progression is 22 and the last term is -11. If the sum is 66, the number of terms in the sequence are:

Answer: Option B

Number of terms = n (let)
First term (a) = 22
Last term (l) = - 11
Sum = 66
Sum of an AP is given by:
$$=\text{Number}\text{of terms}\times$$    $$\frac{\text{First}\text{term}+\text{Last}\text{term}}{2}$$
$$\begin{array}{rl}& 66=\text{n}\times \frac{\text{a}+\text{l}}{2}\\ & 66=\text{n}\times \frac{22-11}{2}\\ & 66=\text{n}\times \frac{11}{2}\\ & \text{n}=\frac{66\times 2}{11}\\ & \text{n}=12\\ & \text{No}\text{.}\text{of}\text{terms}=12\end{array}$$