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Cost for copies is a linear function of the number of copies. If 100 copies cost $16.00,and 125 copies cost $ 18.25, write a formula for a copy cost as a linear function of the number of copies. Then find how much it would cost to make 400 copies.
If x is the number of copies made and y is the amount charged for the copies, what is the formula for the amount.
Thanks for your help. This is a hard one.

Note that 125 copies is 25 more than 100 copies.
The difference in price is $2.25.
Taking 2.25/25 is 9c each.
This means the 100 copies cost $9.

If that is how much 100 copies cost, there is a $7 cost just for using the machine.
That makes the equation for price be y = $7 + $0.09x where x is the number of units.
Using this to check, for x= 100, we get y = $7 + $9 = $16 and
for x = 125, we get y = $7 + $11.25 = $18.25.

If x = 400, we get y = $7 + $36 = $43.  


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Scott A Wilson


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