If p = -1 and x = [ 1-n/n+1 - (1 - n / n+1)] (n+1/n) then px

[lisa.nguyen]

Thank you for taking the time to read my question. I recently took a CAT exam and the question is below:

If p = -1 and x = [ 1-n/n+1 - (1 - n / n+1)] (n+1/n) then px =

-1
0
2/3
1
3/2

For this question, I plugged in 1 and accidentally got the correct answer. How do I know when I am not allowed to just plug in a number and solve? (I am assuming I can't just plug a number in for n, but please correct me if I am wrong in my assumption.)

Thanks!
L.

[tommywallach]

Hey Lisa,

You are welcome to plug in here. You could also just notice that [(1-n)/(n+1)] - [(1-n)/(n+1)] will always equal zero, because you're subtracting a thing from itself. But you can always plug in when you have an expression made up of variables. The danger is on a QC question, where the answer choices mean it's possible that plugging in different things could get different results. There can't be different results here based on what you plug in, because otherwise there couldn't be one correct answer to the question.

Hope that helps!

-t