25q + 10d = 425
q + d = 20
First thing's first, picture 4 quarters. Do you know how much these 4 quarters are worth? Obviously, you'd answer one dollar, but take the time to think about how exactly you came up with this answer.
"Well, it's simple," you might think, "Multiply 0.25 by 4."
Similar to this train of thought, the first equation is formulated by doing exactly that. 'q' represents an unknown number of quarters, and 'd' an unknown number of dimes. We use 25q and 10d, because, well, we're multiplying their value by the number of coins. They're equivalent to 425 because the 20 unknown coins make $4.25 (I converted it to cents to get rid of the pesky decimals, but if you want to keep it as dollars that works too).
Because the equation states that there are 20 coins in total, we know that we can make the equation 'q + d = 20' since there are 20 quarters and dimes in total.
Let me know if you need any further explanation :)
-T.B.
