Given:To find: The greatest integer value of x + y
Solution:Before we see the correct solution, let's discuss a
Common Mistake.
Often, students find the greatest integer values of x and y separately (3 and 6, respectively) and then add them to get 9 as the answer. But this is WRONG!
See why:
- Adding the two given inequalities, we get 5 < (x + y) < 11.
- Thus, x + y is a real number between 5 and 11, exclusive.
- The greatest integer in this range is 10. And thus, greatest integer value of (x + y) is 10.
Observe how this 10 is more than the 9 we got earlier. That 9
assumed that x and y also had to be integers.
In reality, having x = 3.5 and y = 6.5 gives us x + y = 10. (There are many other combinations of x and y that can give us 10.)
Key Learning: Never assume numbers to be integers unless explicitly stated. Assumption is a big killer on the GRE.
Shweta Koshija
GMAT, GRE, SAT Coach for 10+ years