A “double-decker” sandwich has three slices of bread and two layers of filling (bread/filling/bread/filling/bread). Each slice of bread has to be buttered on each side that is in contact with the filling. I make as many of these sandwiches as possible from a slice loaf which has 22 usable slices, excluding the crusts which are not used. How many sides of bread do I have to butter?
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Junior Mathematical Challenge 1999 (1999)
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