Question

Find the sum of all the natural numbers between 1 to 200 which are multiples of 5

Answer 1

4100 will be the answer.

Answer 2

solution:

to find: sum of all the multiples between 1 to 200,which are multiples of 5
∴$a=5$,
∴$d=5$,
∴$a_n=200$

          $a_n=a+(n-1)d$
          $200=5+(n-1)5$
       $200-5=(n-1)5$
          $195=(n-1)5$
       $195/5=n-1$
            $39=n-1$
          ∴ $n=40$

$S_n=n(a+l)/2$
[tex]S_{40}=40(5+200)/2 [/tex]
$S_{40}=20(205)$
∴$S_{40}=4010$

∴The sum of all the natural numbers between 1 to 200, which are multiples of 5 is 4010