[Project Euler]Problem 1 - Add all the natural numbers below one thousand that are multiples of 3 or 5.

將小於1000的3的倍數與小於1000的5的倍數全部加起來，
接著再扣掉全部小於1000的15的倍數即可得解。

[Ruby]

	answer = 0
	3.step(999,3){ |i| answer += i }
	5.step(999,5){ |j| answer += j }
	15.step(999,15){ |k| answer -= k }
	p answer

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