Question

What is the sum of all the multiples of 5 greater than 50 but less than 200?

Answer 1

First multiple after 50 = 55

Last multiple before 200 = 195

therefore, a = 55, d = 5

a_n = 195 

=> 55+(n-1)5 = 195

(n-1)5 = 195-55

(n-1)5 = 140

(n-1) = 140/5

n = 28+1 = 29

therefore, S₂₉ = 29/2 x (55+195)

S₂₉ = 29x250/2

S₂₉ = 29X125

S₂₉ = 3625

Hence, the sum of all the multiples of 5 greater than 50 but less than 200 is 3625