1. ## Re: Postage Stamps Problem?

Look at the last digit:
0 or 5 needs 0x11
1 or 6 needs 1x11
2 or 7 needs 2x11
3 or 8 needs 3x11
4 or 9 needs 4x11.
39 is the biggest number that cannot be reached because 4x11 is 44.

2. ## Re: Postage Stamps Problem?

Originally Posted by kikhakplz
So i still have to do the counting rightay
I was surprised by the fact that u went from 65-76-87-98
I gave you some pseudo code.
Start with your target number $K$.
Do a loop consisting of subtracting $11$ from $K$ until you get a multiple of five.
Note that in the loop if $\left\lfloor {\frac{R}{5}} \right\rfloor = \frac{R}{5}$ then you have a multiple of five.
That stops the loop & gives you the result. If that is still a mystery to you then please find a face to face sit-down help/

3. ## Re: Postage Stamps Problem?

This is the Diophantine Frobenius Problem

If $\displaystyle gcd(a,b)=1$ then the number you are looking for is $\displaystyle m=ab-a-b$

example

$\displaystyle a=5$, $\displaystyle b=11$, $\displaystyle m=39$

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