The average of your 3 quiz grades is 17 points. Two of your quiz grades are 14 points and 19 points. Write and solve an equation to find the score of your third quiz.

Answer 1

The average is found by adding all of the quiz grades and then dividing that total by the number of quizzes:

Average = quiz1 + quiz2 + quiz3

3

Using this formula we can plug in what we are given from the problem:

Average = 17

Total # of quizzes = 3

quiz1 = 14

quiz2 = 19

Since we don't know the score of the third quiz:

quiz3 = x

Now we can plug what we know into the formula:

Average = quiz1+ quiz2 + quiz3

3

17 = 14 + 19 + x

3

Now we must solve for x. First, we multiply both sides of the equation by 3 as follows:

(3) 17 = 14 +19 + x (3)

3

The 3's on the right hand of the equal sign cancel out and we are left with:

51 = 14 + 19 + x

Next, we need to isolate x so that it's by itself on one side of the equal sign. We do this by performing the opposite operation of what is shown in the problem. Because 14 and 19 are both being added to x, we will subtract 14 and 19 from both sides of the equation:

18 = x so the third quiz grade is 18.

You can double check your answer by adding all of the quiz grades together then dividing by three to make sure you get an average of 17:

14 + 19 + 18 = 51

51 = 17

3

Since you get the average of 17 that was given in the problem, you know that your answer of 18 for the third quiz grade is correct.

Answer 2

14+19+x = 3(17) = 51

x = 51-14-19 = 18