Question

What is the sum of all the multiples of 3 and 5 below 1000?

Answer 1

Now you can just put these ingredients together to solve the problem. To find n use 1000/3 = 333 + remainder, 1000/5 = 200 + remainder, 1000/15 = 66 + remainder and then sum multiples of 3: 3⋅333(333+1)/2=166833. multiples of 5: 5⋅200(200+1)/2=100500 and subtract multiples of 15 15⋅66(66+1)/2=33165 to get 234168