Question

Find S20 if the series 1+1.1+... is arithmetic.

A. 2.9
B. 1.9
C. 39
D. 78

Answer 1

Here, first term(a)=1
common difference(d)=1.1-1=0.1
Now, use Sn=n*{2a+(n-1)d}/2
to find S20

Answer 2

Does it help?

Answer 3

I just dont know how to do this.

Answer 4

I got the answer. Is it A?