Stage: 4 Challenge Level: 
The
country Sixtania prints postage stamps with only three values 6
lucres, 10 lucres and 15 lucres (where the currency is in
lucres).Which values cannot be made up with combinations of these postage stamps? Prove that all other values can be made up.
Mathematical reasoning & proof. Dynamical systems. Number theory. Algorithms. Inequality/inequalities. Codes and cryptography. Addition & subtraction. Combinatorics. Stage 5 - Reviewed 2012. Manipulating algebraic expressions/formulae.
Published October 2003.
Copyright © 2007. University of Cambridge. All rights reserved.
NRICH is part of the family of activities in the Millennium Mathematics Project, which also includes the Plus and Motivate sites.
email: nrich@damtp.cam.ac.uk