5 friends, A, B, C, D and E, sit next to each other in 5 adjacent seat

First, let's put A to the left of C. Then we know somewhere in our seating arrangement, we have AXC, where X is a person sitting between A and C. In this case, there are only three choices for where to put person A -- the first, second, or third seats. We can't put A in the fourth or fifth seat, because there's no sixth or seventh seat where we could put person C. So in this case, we have 3 choices for where to put A, after which the position of C is fixed, and then we still have 3 empty seats, and we can seat the remaining people in those seats in (3)(2)(1) = 6 ways. So, multiplying our choices, we have (3)(6) = 18 seating arrangements in total.

But we could also put C to the left of A. We'll again find we have 18 possible seating arrangements. So in total, there are 18+18 = 36 arrangements.