What is the units digit of the number z?

1. 17 < z < 20
z=18 unit=8, z=19 unit=9.
Not sufficient

2. 18 < z < 21
z=19 unit=9, z=20 unit=0.
Not sufficient

(1+2)18<z<20
z=18.5 unit=8, z=19 unit=9.
Not sufficient (it does not say that z is a integer, so z can be a decimal as well).

Z is an integer

St 1: the unit digit will be either 8 or 9. INSUFFICIENT

St 2: The unit digit will be either 9 or 0. INSUFFICIENT

ST 1 & ST 2: The unit digit will be 9. ANSWER

option C

since we are not told that z is an integer hence it can be in decimal.

1. 17 < z < 20
it means:
z can be 17.01===18===18.01===19===19.99
hence possible answers are 7 or 8 or 9
not sufficient

2. 18 < z < 21
it means :
z can be 18.01===19===19.01===20===20.01===20.99
hence possible answers 8 or 9 or 0
not sufficient

combining both
18<z<20
z can be 18.01===19===19.01===19.99
hence possible answers 8 or 9
again not sufficient
not sufficient

hence E

Statement 1
z can be any number (including non-integer) between 17 & 20
Not sufficient

Statement 2
z can be any number (including non-integer) between 18 & 21
Not sufficient

Statement 1 & 2
z can be any number (including non-integer) between 18 & 20
If Z was integer, then option C would be sufficient but nowhere it is mentioned that z is an integer
Thus Not sufficient

Answer E

The key here is identifying the fact that z CAN be a non-integer. hence E.

(1) 17<z<20
z can be 18 or 19 different units digit insufficient.
(2) 18<z<21
z =can be 19 or 20 different units digit insufficient.

(1)+(2)
18<z<20
z can be 18.7 or 19 or 19.8 again different units digit insufficient.

OFFICIAL SOLUTION

Correct Answer: E

The units digit is the digit in the "ones" column; that is, it's the number directly to the left of the decimal point.

Statement 1 indicates tells us that the units digit is 8 or 9 because z could be the integers 18 or 19 and any associated mixed number- we don't know if z is an integer or not. Because we could arrive at multiple answers, this statement is insufficient.

Statement 2 indicates that z could be the integers 19 or 20 (or any associated mixed number), giving us the units digits 9 and 0, respectively. This statement is also insufficient. When taken together, the statements indicate that z must be between 18 and 20, but they do not specify that z must be an integer. For example, z could very well be 19, but it could be 18.5. Because we can still have two different units digits (8 and 9), the statements, even together, are insufficient, and the correct answer is E.
_________________

Please note that the correct answer to this question is E, not C. You can check the OA in the original post and solutions (including official one) above your post.
_________________

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________