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From 1: p/n = 7x/4x
From 2: p/n = 3a/2a
Both put together: 7x - 3a = 3
and the nickels from the 2 statements are 4x = 2a => x = 1/2a
Substituting this value in the equation 7x - 3a = 3 will yield a value for x which can be used to compute the number of nickles.