## GMAT prep Problem- equations

Math questions from mba.com and GMAT Prep software
michalw623
Course Students

Posts: 2
Joined: Sat Apr 09, 2016 6:45 am

### GMAT prep Problem- equations

GMAT prep Problem, from the official Gmat prep

If XY + Z = x(Y+Z) which f the following must be true

A. X=0 and Z=0

B. X=1 and Y=1

C. Y=1 and Z=0

D. X=1 or Y=0

E. X=1 or Z=0

I Checked the answers and both b and C must be true, which obviously cant be...

B- 1 +Z = 1 + Z
C. X +0 = X+ 0

Where am I wrong???

Thanks
tim
ManhattanGMAT Staff

Posts: 5665
Joined: Tue Sep 11, 2007 9:08 am
Location: Southwest Airlines, seat 21C

### Re: GMAT prep Problem- equations

You've ignored the word "must" in the question. By your logic all five answer choices would be correct (try it yourself!).
Tim Sanders
Manhattan GMAT Instructor

https://www.manhattanprep.com/gmat/forums/a-few-tips-t31405.html
RonPurewal
Students

Posts: 19747
Joined: Tue Aug 14, 2007 8:23 am

### Re: GMAT prep Problem- equations

yeah... "must be true" DOES NOT mean "if you plug it in, it works".

"must be true" means "it's IMPOSSIBLE for this to be FALSE".
Students

Posts: 88
Joined: Tue Jan 26, 2010 6:47 am

### Re: GMAT prep Problem- equations

Given: XY + Z = X(Y+Z)

Let us expand:

XY+ Z = XY+ XZ

XY cancels out.

Z = XZ

or XZ-Z= 0
Z*(X-1) = 0

so either Z=0 or X = 1