If 65 percent of a class answered...

[A4Fever]

the first question on a certain test corectly, 45 percent answered the second question on the test correctly, and 10 percent answered neither of the questions correctly, what percent answered both correctly?

This is from OG 10th edition however I did change the numbers in order to post on the forum. Hope this is ok.

My question- I set up the double set matrix but can't seem to be able to answer this one. Can someone set one up & explain how they got to their answer?

Thanks
A4Fever

[JonathanSchneider]

You might be forgetting to simply add "100" to the box representing the overall total. Since we are just dealing with percents, not real numbers, you can do that.

[ata]

For simple q's like this think its a much better idea to use the formula approach than the table.

Total = GRP A + GRP B - BOTH + NEITHER

As per values posted by you:

1 = 0.65 + 0.45 - x + 0.1
x = 0.2 or 20%

[esledge]

Some people like the formula, some like the double-set matrix. Whatever works reliably for you.

I personally like the matrix, as I have a place for everything/everybody, it requires less mental "keeping track," it is easy to sum vertically and horizontally, and seems more reliable on problems with tricky wording.

That said, for the matrix to work properly, you must label your columns and rows correctly. Maybe your trouble lies here.

Always remember that your two columns should be opposites (or mutually exclusive), and the same should be true of the rows. One way to do so is to name a category/quality for both columns at a whole, then label the individual columns with Yes and No.

Note that First Question and Second Question are not opposites! First Question Right and First Question Wrong are opposites! (Equivalently: "First Question Right?" as the column header, and a Yes above one column, a No above the other column)

Hope this helps!