If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.Find the sum of all the multiples of 3 or 5 below 1000.

Given :

All the natural numbers below 1000 that are multiples of 3 or 5 .

To Find :

The sum of all the multiples of 3 or 5 below 1000.

Solution :

Max multiple of 3 is 999 .

Max multiple of 5 id 995 .

So , number of multiple of 3 is :

Similarly for 5 .

Now , sum of all multiple of 3 is given by :

Also , sum of all multiple of 5 is :

Therefore , total sum :

Now , there are some common number which we add two times like :

15 , 30 , 60 ......

So , we should subtract the sum of all multiple of 15 from T .

Now , sum of all multiple of 15 is 33165 .

So ,

Therefore , the  sum of all the multiples of 3 or 5 below 1000 is 233168 .