Astudent got a grade of 85% on a text in which she answered 3 questions incorrectly. how many questions were on the test?

85% right, 15% wrong

15%            3
   =       then cross multiply 3x100=15x?
100%           ?

300=15x?
? = 20 questions

Alright, we have to ask ourselves "How much of 100% is one question worth?"

To find this, we must first find out how much was taken off of 100% to get 85%.  In terms of math, this can be put like this:

100% - X = 85%

Now, we solve for X.

100% - X = 85%

Add X to both sides.

100% = 85% + X

Subtract 85% from both sides.

100% - 85% = X

Subtract.

15% = X

This means that 15% was taken off of 100% to get 85%.  In other words, 3 incorrect questions are equal to 15% of the final grade.  In terms of math, we can put it like this:

3 = 15%

Now, let's go back to the first question we asked at the beginning.

"How much of 100% is one question worth?"

To find this, we need to make 3 become 1 in the equation 3 = 15%.

3 = 15%

Divide both sides by three.

3/3 = 15%/3

Divide.

1 = 5%

This means that 1 question is worth 5% of the grade.  With this information, we can find out how many questions it takes to get 100% of the grade.  To do this, we need to make the 5% become 100%.

1 = 5%

Multiply both sides by 20.

20(1) = 20(5%)

20 = 100%

This means that there are a total of 20 questions on the test.