Question

Find sum of all natural numbers from 1-100 . Which are divisible by 2 and 5

Answer 1

the sum is equal to 550

Answer 2

10 , 20 ,........................,90

a = 10

d= 20-10

= 10

tn = a + (n-1)d

90 = 10 + (n- 1) 10

80 = (n-1)10

n= 9

Sn = n/2(a+L)

= 9/2(10 +90)

= 9/2×100

= 9×50

= 450