Question:
The first and last terms of an A.P. are 1 and 11. If the sum of its terms is 36, then the number of terms will be
(a) 5
(b) 6
(c) 7
(d) 8
Solution:
In the given problem, we need to find the number of terms in an A.P. We are given,
First term (a) = 1
Last term (an) = 11
Sum of its terms 
Now, as we know,
$S_{n}=\left(\frac{n}{2}\right)(a+l)$
Where, a = the first term
l = the last term
So, we get,
$36=\left(\frac{n}{2}\right)(1+11)$
$36(2)=12 n$
$n=\frac{36(2)}{12}$
$n=6$
Therefore, the total number of terms in the given A.P. is $n=6$
Hence the correct option is (b).