<PRE><B>Question 165667</B>
{{{(75+83+77+81+2x)/6 =80 }}} Start with the given equation



{{{(316+2x)/6 =80 }}} Combine like terms.



{{{cross(6)((316+2x)/cross(6)) =80(6) }}} Multiply both sides by 6 



{{{316+2x=480}}} Multiply



{{{2x=480-316}}} Subtract {{{316}}} from both sides.



{{{2x=164}}} Combine like terms on the right side.



{{{x=(164)/(2)}}} Divide both sides by {{{2}}} to isolate {{{x}}}.



{{{x=82}}} Reduce.



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Answer:


So the answer is {{{x=82}}} which means that he must an 82 or better to get an average of 80 or better.</PRE>