Final answer:
Sally has 24 nickels.
Step-by-step explanation:
To determine the number of nickels Sally has, we can set up a system of equations using the given information. Let's assume Sally has x nickels and (42 - x) dimes. The value of the nickels is 5x cents, and the value of the dimes is 10(42 - x) cents. We can set up the equation: 5x + 10(42 - x) = 300 (since the total value is $3.00).
Simplifying the equation, we get: 5x + 420 - 10x = 300. Combining like terms, we have: -5x + 420 = 300. Subtracting 420 from both sides gives us: -5x = -120. Dividing both sides by -5, we find x = 24.
Therefore, Sally has 24 nickels.