Question 847420
Let 

x = number of $1 bills
y = number of $5 bills


There are 200 bills of just $1 and $5 bills. So the two must add to 200


x + y = 200


Solve for y to get y = 200 - x


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If you have x $1 bills and y $5 bills, then you'll have a total of x + 5y dollars. But we know this total is also equal to $300, so


x + 5y = 300


Replace each copy of y with 200 - x (since above we know that y = 200 - x) to get


x + 5<font color="red">y</font> = 300


x + 5(<font color="red">y</font>) = 300


x + 5(<font color="red">200 - x</font>) = 300


x + 5(200 - x) = 300


Notice how y is completely gone now. Now solve for x


x + 5(200 - x) = 300


x + 5(200) + 5(-x) = 300


x + 1000 - 5x = 300


-4x + 1000 = 300


-4x = 300 - 1000


-4x = -700


x = -700/(-4)


x = 175


We could stop here, but let's keep going to find y


y = 200 - x


y = 200 - 175


y = 25


This bit is extra, but it helps to know how to find it (if asked).


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Answer:


Since x = 175, this means that there are <font size = 5 color="red">175</font> one dollar bills.


Note: if you forget what x is, look back up at the definitions above. This is why it helps to set these up first.