How do you solve this puzzle?

September 17, 2009 6:59 AM

Please help me understand how to duplicate the solution to this puzzle: What is the largest amount of postage you cannot make exactly with 7- and 11-cent stamps? (Spoiler inside, so don't look if you want to figure it out yourself - I have the solution but would like to learn how it was solved.)

So apparently the solution is 59 cents. But how would you figure that out without trying every combination manually? I feel like there's something really obvious that I'm missing.

Thx!

posted by widdershins to grab bag (5 comments total)

http://www.willamette.edu/people/archives/2007/09/math_professors.html
posted by decathecting at 7:07 AM on September 17, 2009

I found that, BTW, by googling the part of your question that is bolded. It's the 6th google result.
posted by decathecting at 7:08 AM on September 17, 2009

try this link: http://mathforum.org/library/drmath/view/65669.html
posted by gushn at 7:09 AM on September 17, 2009 [1 favorite]

gah. I didn't think to google the whole question. *hangs head*

So for posterity, the formula is: Take the product of the two stamp amounts (7 x 11 = 77) and subtract their sum (7 + 11 = 18): 77 - 18 = 59.

Thank you decathecting...
posted by widdershins at 7:12 AM on September 17, 2009

does anybody have a proof?
posted by wayofthedodo at 3:59 PM on September 18, 2009 [1 favorite]