WEBVTT

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Here is the problem: The average of 70, 80, 90 and x is 85. Find x.

2
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The mean of 70, 80, 90 and x is 85.

3
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Total needed. 4 values with mean 85 sum to 4 times 85 equals 340.

4
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Solve for x. x equals 340 minus (70 plus 80 plus 90) equals 340 minus 240 equals 100.

5
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So the answer is x equals 100.

6
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The mean of 70, 80, 90 and x is 85.

7
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(70 plus 80 plus 90 plus 100) / 4 equals 85, which checks out.