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Explanation:
It costs $91.50 for the first 200 copies.
Roger needs 200 more copies to get to 400 total.
He's charged $42 per 100 copies at this point, so he is charged 42*2 = 84 extra dollars to do those extra 200 copies.
Before tax, his total is 91.50+84 = 175.50 dollars.
The sales tax will increase the bill by 6%
To increase by 6%, we use the multiplier 1.06
1.06*175.50 = 186.03
Or you can follow the steps shown in the next section.
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6% of 175.50 = 0.06*175.50 = 10.53
The sales tax is $10.53
175.50 + 10.53 = 186.03
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Whichever method you use, you should end up with an after-tax cost of $186.03