Project Euler - Problem 1 Multiples of 3 and 5 (Java)
You used the inclusion-exclusion principle here…


sumOfMultiples(3,limit) + sumOfMultiples(5,limit) - sumOfMultiples(15,limit);



… which is good.  The name of the function sumOfMultiples(…) is pretty clear.  (The only uncertainty is whether it includes or excludes the upper bound.  JavaDoc would help in that regard.)

There is a way to compute sumOfMultiples() in constant time.  Since

$$ 1 + 2 + 3 + \ldots + n = \frac{n(n+1)}{2}$$

you can derive a formula for

$$ k + 2k + 3k + \ldots + nk = k \frac{n(n+1)}{2} $$