1) Multiples of 3 and 5

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.

by Aarushi Kansal

JavaScript

//Arithmetic progression 1 + 2 + 3 + … + n = (n * (n + 1)) / 2
function sumDivisibleBy(n) {
    var p = parseInt(999 / n, 10);
    return parseInt(n * (p * (p + 1)) / 2, 10);
}

var sum = sumDivisibleBy(3) + sumDivisibleBy(5) - sumDivisibleBy(15);
console.log(sum); //233168