Omar's test scores are 72, 78 and 81. For a mean of 80, what score…

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The question

Question: Omar's test scores are 72, 78 and 81.

For a mean of 80, what score does Omar need on the next test?

1 Total needed

After the next test there will be 4 scores.

For a mean of 80, the 4 scores must add up to 80 times 4.

That is 320.

The mean times how many numbers gives the total.

That is the key idea today.

Careful!

With the next test, there are 4 scores, not 3.

So we multiply by 4.

2 Total so far

Now we add the 3 scores we know, from left to right.

So far, the total is 231.

3 What is missing

The total must go from 231 up to 320.

The missing part is 320 minus 231.

That is 89.

4 Check

Let's check: 231 plus 89 is 320.

And 320 divided by 4 is 80, the mean we wanted.

So it is right!

Let's look back.

First: total needed.

Then: total so far.

Then: what is missing.

Last: check.