so we have; term a26=0 , term a11=3 and last term an=-0.2 ( -1/5 = -0.2)
let common difference is d we have d=(a26-a11)/(26-11) substituting the values we get;
(0-3)/(26-11) -->>> -3/15 = -0.2 so common difference d=-0.2
Now lets calculate the number of terms n ( it is obvious that we can not have a negative number of a term.
so we can not have n=-25 . number of terms are possitive numbers)
so we have an=a26+(n-26)(-0.2) {the -0.2 is the common difference we have found before}
subsituting the values we have; -0.2=0+(n-26)(-0.2) -->>> -0.2=(n-26)(-0.2)
deviding by -0.2 both sides of the equation we get;
1=n-26 solving for n we get; n=26+1 so n=27