Mo2men wrote: Can a multiple be negative?

Yes, it can. It can also be zero.

So, for example, the multiples of 11 are: . . . -44, -33, -22, -11, 0, 11, 22, 33, 44, 55...

Cheers,
Brent

Mo2men wrote:Is b < 0?

(1) Integer b is a multiple of 13
(2) b³ < 13

Target question: Is b less than 0?

Statement 1: Integer b is a multiple of 13
There are several values of b that satisfy statement 1. Here are two:
Case a: b = -13, in which case x IS less than 0
Case b: b = 0, in which case x is NOT less than 0
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: b³ < 13
There are several values of b that satisfy statement 2. Here are two:
Case a: b = -13, in which case x IS less than 0
Case b: b = 0, in which case x is NOT less than 0
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
IMPORTANT: Notice that I was able to use the same counter-examples to show that each statement ALONE is not sufficient. So, the same counter-examples will satisfy the two statements COMBINED.
In other words,
Case a: b = -13, in which case x IS less than 0
Case b: b = 0, in which case x is NOT less than 0
Since we cannot answer the target question with certainty, the combined statements are NOT SUFFICIENT

Answer: E

Cheers,
Brent

Mo2men wrote: Dear Brent,
Thanks for your repose but my question maybe was not clear.

Does GMAT deals with multiples in negative? I have seen it only in positive forms. Likewise, in dealing with factors, we deal with only positive numbers and I have not seen it in Negative form.

My concern is that GMAT does not accept some forms, while mathematically it is correct.

From the Official Guide
If x and y are integers and x ≠ 0, then x is a divisor (factor) of y provided that y = xn for some integer n. In this case, y is also said to be divisible by x or to be a multiple of x. For example, 7 is a divisor or factor of 28 since 28 = (7)(4), but 8 is not a divisor of 28 since there is no integer n such that 28 = 8n

So, as you said, divisors and multiples can be negative.

You're also right in that official GMAT questions restrict the divisors and multiples to positive values.

Cheers,
Brent