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**relyt** Show that any amount of postage of at least 12 cents can be made using 3 cent and 7 cent stamps?

I tried a couple of numbers of 12 and greater:

P(12) is true. 12 = 3 + 3 + 3 + 3.

P(13) is true. 13 = 7 + 3 + 3.

P(14) is true. 14 = 7 + 7.

P(15) is true. 15 = 3 + 3 + 3 + 3 + 3.

P(16) is true. 16 = 3 + 3 + 3 + 7

P(17) is true. 17 = 7 + 7 + 3

P(18) is true. 18 = 3 + 3 + 3 + 3 + 3 +3

P(19) is true. 19 = 3 + 3 + 3 + 7 + 3

and I can see that you can form postage of x + 1 by using postage of x - 3 plus an additional 3 cent stamp....but how do I show this to solve the problem?

Any help would be appreciated