Question

The sum of all integers between $81$ and $719$ which are divisible by $5$ is

• A. $51800$
• B. $50800$
• C. $52800$
• D. None of these

Answer 1

The correct option is B $50800$

All integers between $81$ and $719$ which are divisible by $5$ form an AP with common difference $5$.

First term $a=85$ , last term $l=715$ and common difference is $d=5$

Then, apply the formula for $n^{th}$ term of an AP

$715=85+(n-1)5$

$126=n-1$

$\Rightarrow n=127$

To get the required sum, apply the formula for sum of $n$ terms of an AP

$S_n=\frac{n}{2}(a+l)=\frac{127}{2}(85+715)=50800$