<PRE><B>Question 1009971</B>
x = number of problems on the test.


4 problems wrong means x-4 problems right.


(x-4)/x = .8


multiply both sides of this equation by x to get x-4 = .8x.


subtract .8x from both sides of this equation and add 4 to both sides of this equation to get x - .8x = 4


combine like terms to get  .2x = 4


divide both sides of this equation by .2 to get x = 4/.2 = 20.


there were 20 problems on the test.


he mised 4 so he got 16 right.


16/20 = 8/10 = .8 * 100 = 80%


you could also have just figured out that if he got 80% right, then he must have gotten 20% wrong.


20% wrong = 4


x = total on test.


.20 * x = 4


divide both sides by .20 and you get x = 4/.2 = 20.


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