Question

Find the sum of all whole numbers divisible by $5$ but less than $100$.

• A. $950$
• B. $925$
• C. $880$
• D. $1050$

Answer 1

The correct option is A $950$

All whole numbers divisible by $5$ but less then $100$ are
$0,5,10,15,\mathrm{20........95}$
It is an A.P. with first term, common difference and last term as $0,5$ and $95$ respectively.
$a=0,d=5,a_n=95$
$a_n=a+(n-1)d$
$95=0+(n-1)5$
$5n=100$
$n=20$
Hence total numbers are $=20$
Sum of numbers are
$S_n=\frac{n}{2}[a+(n-1\left)d\right]$

$S_{20}=\frac{20}{2}[0+(20-1\left)5\right]$

$S_{20}=10\times 95=950$
The sum of all natural number divisible by $5$ but less than $100$ is $950$.