Question

The sum of all $2$-digit numbers divisible by $5$ is _________?

• A. $1035$
• B. $1245$
• C. $1230$
• D. $945$

Answer 1

The correct option is D $945$

The series will be : 10, 15, 20,.....,90,95

$S_n=10+15+20+......+90+95$

here a will be first term

a = 10

& d = difference b/w any

two terms

d = 5

series containing n terms

& n = $1+\frac{(Lastterm-firstterm)}{d}$

$=1+\frac{(95-10)}{5}$

n = 1 + 17 = 18

now, $S_n=\frac{n}{2}[2a+(n-1)d]$

$=\frac{18}{2}[2\left(10\right)+(18-1)\left(5\right)]$

$=9(20+85)$

$S_n=9\left(105\right)$

$Sn=945$