Marissa has 24 coins in quarters and nickels. She has 3 dollars. How many of the coins are quarters?

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Let n be the number of nickels and q be the number of quarters.

We have two equations:
(1) n + q = 24
(2) 0.05n + 0.25q = 3

Rearrange (1) to solve for n in terms of q for another equation (3)
(3) n = 24 - q

Plug (3) into (2)
0.05(24 - q) + 0.25q = 3

Multiply through:
1.2 - 0.05q + 0.25q = 3

Combine q terms
0.2q + 1.2 = 3

Subtract 1.2 from each side:
0.2q = 1.8

Divide each side by 0.2
q = 9