Question

Find the sum of all natural numbers less than 100 which are either divisible by 7 or 5

Answer 1

a = 5

d =5

l= 100

an = a+(n-1)d

100 = 5 + (n-1)d

95÷5 =n-1

n = 20

Sn = n÷2 (a + l)

Sn = 10 (5 + 100)

Sn = 1050