F(x) = (x + 1)(x - 2)
f(x) = x(x - 2) + 1(x - 2)
f(x) = x(x) - x(2) + 1(x) - 1(2)
f(x) = x² - 2x + x - 2
f(x) = x² - x - 2
ANSWER: 32 five-dollar bills
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EXPLANATION:
Let x be number of $5 bills
Let y be number of $10 bills
Since we have total of 38 bills, we must have the sum of x and y be 38
x + y = 38 (I)
Since the total amount deposited is $220, we must have the sum of 5x and 10y be 220 (x and y are just the "number of" their respective bills, so we multiply them by their value to get the total value):
5x + 10y = 220 (II)
System of equations:

Divide both sides of equation (II) by 5 so our numbers become smaller

Rearrange (I) to solve for y so that we can substitute into (II)

Substituting this into equation (II) for the y:

We have 32 five-dollar bills
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If we want to finish off the question, use y = 38 - x to figure out number of $10 bills

32 five-dollar bills and 6 ten-dollar bills
This is middle school??? AinT no way
I think the answer is 9.375 if it needs to be in decimal form. If it needs to be in fraction form it would be improper, 9 3/8 would be the fraction. Hope it helps!
There has to be something in between the two numbers, for example 7 x 7 or 7+7 not just 7 7. Without the equation it would be highly impossible to answer :D