A={1,2,3,4,5…...n}
No. of subsets having r elements such that no two are consecutive is =n−r+1Cr
for n=5, no. of ways =6⋅rCr
Subsets having no element =1
Subsets having exactly 1 element =5C1=5
Subsets having exactly 2 element =4C2=6
Subsets having exactly 3 element =3C3=1
⇒5+6+1+1=13