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Investigation: Planks Stage: 3 and 4 I have an unlimited supply
of planks, of lengths 7 and 9 units. Putting planks end to end,
what total lengths can be achieved? For example, I can make a
length of 30 by using three planks of length 7 units and one of
length 9 units, but what about a total length of 40? The Excel
file shows all the possible combinations: I usually right-click
on the link, choose "Save Target As", and select where I want
the file to be saved. In one direction (down) are the multiples
of 7, including 0, and in the other direction (across) are the
multiples of 9. The numbers in t