What is the average of all numbers between 11 and 80 which are divisible by 6?

Answer: Option D

$$\begin{array}{rl}& \text{11 to 80 divisible by 6}\\ & \text{12, 18, }\text{. }\text{. }\text{. }\text{. }\text{. }\text{. }\text{. }\text{. }\text{. 78}\\ & \text{First term}\left(a\right)=12\\ & \text{Last term}\left(l\right)=78\\ & \text{Total number of terms}\\ & =\frac{\text{Last term }-\text{ First term}}{\text{Common difference}}+1\\ & =\frac{78-12}{6}+1\\ & =12\\ & \text{Average of 12 numbers}\\ & =\frac{n}{2}\frac{\left(a+l\right)}{n}\\ & =\frac{12+78}{2}\\ & =45\end{array}$$