Suppose you have just enough money, in coins, to pay for a gallon of milk priced at $2.40. you have 12 coins, all quarters and dimes. let q equal the number of quarters and d equal the number of dimes. which system models the given information?

The answer is
q + d = 12
0.25q + 0.10d = 2.40

q - the number of quarters
d - the number of dimes

You have 12 coins, all quarters and dimes:
q + d = 12

The value of 1 quarter is $0.25.
The value of 1 dime is $0.10.
Therefore, if you want to pay $2.40 you will have q quarters of value $0.25(0.25q) and d dimes of value $0.10d (0.10d).
The second equation is:
0.25q + 0.10d = 2.40