Factor Foundation Rule & Prime Factorization: HELP!

[Ineedhelp]

I don't understand this question, pleas help.

Positive Integers A and B both have exactly four factors. If A is a one digit number and B = A+9, what is A?

Please explain, thanks.

[hiteshwd]

Sure

Given: A & B both have four factors, A is single digit no., B=A+9

Solution:
Think of values that A can have as it can lie only between 1 and 9
The only values between 1 and 9 that have four factors are 6 & 8
So A can be either 6 or 8

=> B can be either 15 or 17, but only 15 has four factors

hence sol is A= 6

[StaceyKoprince]

Please read (and follow!) the forum guidelines before posting.

This folder is only for general strategy questions, not content or specific test problems. Check out the content / problem folders and post in the relevant folder depending upon the source of the problem you want to post (and make sure to follow the rules about banned sources). If you are a course student, you can also ask about other problems or issues before or after class or during section.

hiteshwd, thanks for responding to a fellow student - nice explanation!

[azama88]

The question asks "Positive Integers A and B both have exactly four factors" rather than stating four non-negative or positive factors.
So, why we are not considering negative factors here?

[StaceyKoprince]

Factors are considered positive by definition.

(There's probably some advanced math in the world that gets into negative factors...but the GMAT only tests high-school-level math—so factors are positive.)