Question

Find the sum of all numbers lying between 201 and 424 which are divisible by 5.

Answer 1

First term a = The first number after 201 divisible by 5 is 205.

Last term (l) = The last number before 424 divisible by 5 is 420.

d = 5

l = a + (n − 1)d

420 = 205 + (n − 1) 5

420 − 205 = (n − 1) 5

215 = (n − 1) 5

`215/5 = (n − 1)`

43 = n − 1

43 + 1 = n

n = 44

`S_n = n/2 (a + l)`

`S_44 = 44/2 (205 + 420)`

= `22 × 625 `

= 13,750