Let's make a system of equations to solve for this:
n + q = 84
0.05n + 0.25q = 12.80
where n = nickels and q = quarters.
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Solve for n in the first equation by subtracting q from both sides.
n = 84 - q
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Plug n into the second equation.
0.05(84 - q) + 0.25q = 12.80
Distribute 0.05 inside the parentheses.
4.2 - 0.05q + 0.25q = 12.80
Combine like terms.
4.2 + 0.2q = 12.80
Subtract 4.2 from both sides.
0.2q = 8.6
Divide both sides by 0.2.
q = 43
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Plug 43 for q in the first equation.
n + (43) = 84
Subtract 43 from both sides.
n = 41
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There are 41 nickels and 43 quarters in the piggy bank. ✅