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Sum of all multiples of 3 upto 50
= 3 + 6 + ..... + 48
= 3 (1 + 2 + 3 + .... + 16)

=	3 × 16(16 + 1)	= 3 × 8 × 17
	2

= 408

	∵ 1 + 2 + 3 + .....+ n =	n(n + 1)
		2