Brent Hanneson wrote:Nice, work - the answer is A
Here's another way to tackle this question:

For convenience, we’ll label the two equations:
(a) x + 3y + 5z = 31
(b) x + 5y + 9z = 59

If we subtract (equation b) – (equation a) we get: 2y + 4z = 28

The next part is tricky. We need to recognize that “x + y + z” and “2y + 4z” are both “hiding” in equation (a)

We can rewrite equation (a) as: x + y + z + 2y + 4z = 31
It’s even clearer if we rewrite equation (a) as: (x + y + z ) + (2y + 4z ) = 31

We then replace 2y + 4z with 28, we get (x + y + z ) + (28) = 31

From this, we can conclude that x + y + z = 3

The correct answer is A

Brent Hanneson wrote:If x + 3y + 5z = 31, and x + 5y + 9z = 59, then x + y + z =
A) 3
B) 5
C) 7
D) 9
E) 11

Please note that this is not an official GMAT question; it’s my attempt to create difficult (650+ level) GMAT-style questions for this forum.