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.

Inspired by the Bret Vector's interactive demos, I just wanted to create the solution involving some interaction. So here is the Mathematica code-

Manipulate[Total[Select[Range[x], Mod[#, 3] == 0 || Mod[#, 5] == 0 &]], 
{{x, 1, Style["Number", FontFamily -> "Myriad", 12]}, 1, 999, 1, Appearance -> "Open"}, 
Alignment -> Center, BaseStyle -> {FontFamily -> "Avenir Light", 36}, FrameLabel -> Style["Sum", 12]]

You will require Wolfram CDF player plugin [download here] to view this demo.