As long as we select a value of $b$ that does not contradict any information in the statement 1 or in the main stem, then we are good. Our constraints are that $b$ must be positive and it must be an integer, and the example of $b=3$ and $a=-1$, satisfies all of the conditions. Statement 1, which says $a=b-4$, gives you a relationship between $a$ and $b$ but does not constrain the value of $b$ to a unique integer. For example, I could have picked $b=4$ and $a=0$, and this would have also answered the question as NO.

Does this make sense? Let me know if there anything here that is not clear in my explanation.

Shariful Islam says

Per your explanation in statement one, we have found b = 2. It sounds ok because the value has come from the statement.

But how do you let b = 3 and a = -1? Didn’t the statement restrict the value of b? How can we let other values of b?

Could you please explain related concepts? Thanks in advance for your answer!

GMAT Quantum says

Hi Shariful,

As long as we select a value of $b$ that does not contradict any information in the statement 1 or in the main stem, then we are good. Our constraints are that $b$ must be positive and it must be an integer, and the example of $b=3$ and $a=-1$, satisfies all of the conditions. Statement 1, which says $a=b-4$, gives you a relationship between $a$ and $b$ but does not constrain the value of $b$ to a unique integer. For example, I could have picked $b=4$ and $a=0$, and this would have also answered the question as NO.

Does this make sense? Let me know if there anything here that is not clear in my explanation.

Dabral