Answer:
3 dimes and 10 quarters
Explanation:
If you add 7 dimes, then the number of dimes and quarters is the same, and the total increases to $3.50. A dime and a quarter have a value together of $0.35. Since there are equal numbers, the value $3.50 must be made up of some quantity of groups with a value of $0.35. That number of groups must be ...
... $3.50/$0.35 = 10
Thus, there are 10 quarters and 10-7 = 3 dimes.
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Using an equation
Let q represent the number of quarters. Then the amount Bess has is ...
... 0.10(q -7) +0.25q = 2.80
... 0.35q - 0.70 = 2.80
... 0.35q = 3.50 . . . . . add 0.70 . . . . . . . this should look familiar
... q = 3.50/0.35 = 10