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If Skyler has 15 nickels and quarters in his pocket, and they have a combined value of 235 cents, how many of each coin does he have?
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You can solve it by reducing to the system of 2 equations in 2 unknowns
n + q = 15, (1)
5n + 25q = 235. (2)
n = # of nickels, q = # of quarters.
Or, you can solve it by reducing to the single equation in 1 unknown
5n + 25*(15-n) = 235. (3)
Can you complete the solution from this point on your own?
By the way, if you decide to solve the system (1),(2) by the Substitution method, you will come to the same equation (3).