SOLUTION: On the first 3 exams of his geometry class, Luis got an average score of 81. What does he need on the next exam to have an overall average of 83?

Question 825653: On the first 3 exams of his geometry class, Luis got an average score of 81.
What does he need on the next exam to have an overall average of 83?
Answer by math-vortex(648) (Show Source): You can put this solution on YOUR website!

Hi, there--

THE PROBLEM:
On the first 3 exams of his geometry class, Luis got an average score of 81. 
What does he need on the next exam to have an overall average of 83?


A SOLUTION:
Let x be the score Luis needs on his next exam.

The formula for finding the average score is 

[the sum of the scores] / [the number of scores] = [average score]

Luis had an average of 81 after 3 exams, so we could work backwards from the average 
score to find the sum of his first three scores. We multiply 81*3=243.

The sum of his first three scores is 243. To find his average score after 4 exams, Luis
will add his 4th exam score to 243 and divide the sum by 4. Luis wants an overall score of 83.

Let's write an equation for that.

243 + x is the sum of the first three score plus the score needs on his 4th exam.

We divide by 4 because we are taking the average of 4 scores.

Luis needs his average to equal 83.

(243+x)/4 = 83

Solve for x. Multiply both sides by 4.

243 + x = 83 * 4
243 + x = 332

Subtract 243 from each side.
x = 332 - 243

Simplify.
x = 89

Luis needs a score of 89 on his 4th exam.

Hope this helps!

Mrs. Figgy
math.in.the.vortex@gmail.com

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