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Which of the signals would look the same from the front and the
back?
Can you find pairs of signals that 'match' when seen from in front
or behind?
What about the order in which the signals are sent? Do they
matter?
Place the numbers 1, 2, 3,..., 9 one on each square of a 3 by 3 grid so that all the rows and columns add up to a prime number. How many different solutions can you find?