Pre-Calculus Help! Rational Function (quadratic)?

Posted on 2015-11-18 06:43:00 in Math by LAILANIE

Isobel is producing and selling cassette tapes of her rock band. When she had sold 10 tapes, her net profit was $5. When she had sold 20 tapes, however, her net profit had shrunk to $1 due to increased production expenses. But when she had sold 30 tapes, her net profit had rebounded to $5.

a) Give a quadratic model relating Isobel's net profit y to the number of tapes sold x.
y=

b) Divide the profit function in part (a) by the number of tapes sold x to get a model relating average profit w per tape to the number of tapes sold.
w=

c)How many tapes must she sell in order to make $1.08 per tape in net profit? (Enter your answers as a comma-separated list. Round your answers to the nearest whole number.)
____ tapes

I tried doing it and got this equation but it was wrong,
y=0.04x^2 - 0.625x + 7.25

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