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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
