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[ptraye]

60% of survey respondents completed the survey in 30 minutes or less, but only 30% of survey respondents completed the survey in less than 15 minutes.

We can infer that...

A) 30% of survey respondents completed the survey in 15 to 30 minutes.

B) Only 10% of survey respondents completed the survey in more than 30 minutes.

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I chose A, which is correct, but I'm not sure why it is right. I didn't like B more than I disliked A.

Why is A correct?

[tommywallach]

Hey Ptraye,

If 60% of people completed the survey in 30 minutes or less and 30% finished in less than 15 minutes, we know that the 30% must be contained WITHIN the 60% (because less than 15 minutes is contained within 30 minutes or less). That means the other 30% must have finished between 15 and 30 minutes.

As for (B), it hopes we think that 60 + 30 = 90, leaving 10% for the folks who took over 30 minutes. But remember, that 30% is contained within the 60%, so actually we still have 40% left who finished in over 30 minutes.

-t

[ptraye]

Tommy,

I understand the problem with B, but I still don't understand why A is correct.

Why is a range of 15-30 minutes alright?

Thanks.

[tommywallach]

15-30 is the part of the 0-30 group that isn't 0-15 (we already have information about both 0-30 and 0-15, so whatever's left must be the 15-30 group).