<PRE><B>Question 818724</B>
the following answer is fundamentally for the same problem. 
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simply substitute 639 in the following answer with 500 (i.e. 500 is the lower end of the C scale in your problem.)
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http://www.algebra.com/algebra/homework/Average/Average.faq.question.818730.html
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Solve and graph linear equations:
https://sooeet.com/math/linear-equation-solver.php
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Solve quadratic equations, quadratic formula:
https://sooeet.com/math/quadratic-formula-solver.php
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Convert fractions, decimals, and percents:
https://sooeet.com/math/fraction-decimal-percent.php
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Calculate and graph the linear regression of any data set:
https://sooeet.com/math/linear-regression.php</PRE>