Question

Find the sum of all natural numbers that are less than 100 and divisibile by 4.

Answer 1

The number less than 100 and divisible by 4 are 4, 8, 12, ..... 96.
This will form an AP.
$a_n=96$
a= 4
d =4
$a_n=a+(n-1)d$
96 = 4 + (n - 1) 4
96 = 4 + 4n - 4
$n=\frac{96}{4}=24$
Sum of n terms = $\frac{n}{3}\times (a+l)$
Sum $=\frac{24}{2}[4+96]$
= 12[100]
= 1200