Lena's test scores are 80, 85 and 90. What score on the next test…

Step by step

The question

Question: Lena's test scores are 80, 85 and 90.

What score on the next test makes her mean 87?

1 Total needed

After the next test there will be 4 scores.

For a mean of 87, the 4 scores must add up to 87 times 4.

That is 348.

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 255.

3 What is missing

The total must go from 255 up to 348.

The missing part is 348 minus 255.

That is 93.

4 Check

Let's check: 255 plus 93 is 348.

And 348 divided by 4 is 87, 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.