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.
Addition & subtraction. Manipulating algebraic expressions/formulae. Mathematical reasoning & proof. Visualising. Number theory. Combinatorics. Inequality/inequalities. Generalising. Dynamical systems. Patterned numbers.
Published October 2003.
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