Question 896556
First add all the received scores:
{{{72+85+53+80=290}}}
Let x be the score of the fifth quiz.
We know the student wants an average of 70 for all 5 quizzes, yet he has only taken 4 quizzes.
(Average of 5 quizzes)=(total score of all 5 quizzes)/(# of quizzes)
When you plug in the values you know it will look like this:
{{{70=((290+x)/5)}}}
Solve for x:
Multiply each side of the equation by 5.
{{{70*5=((290+x)/5)*5}}}
{{{350=290+x}}}
Subtract 290 from each side of the equation.
{{{350-290=(290+x)-290}}}
{{{60=x}}}

The student will need a 60 on his fifth quiz to achieve an average of 70 for all 5 quizzes.

-Alejandro