I am new to GMAT and I was completely helpless because of this question. However, GMAT official explanation satisfied me. Thanks to
OG guide. My understanding to the problem:
The Question says units digit of n is greater than 2 so lets assume the number be 3,4,5,6,7,8,9.
Now statement 1 says digit n should equal to n^2's units digit
Lets try with 5 and 6
n^2=5^2=25 where digit 5 is equal to its squared units digit 5.
n^2=6^36 where digit 6 is to its squared units digit 6
HOWEVER, there is no actual answer i.e. whether to choose 5 or 6. So, it is insufficient.
Similarly, Statement 2 says n should equal to n^3's unit digit
Lets try with 4 and 5
n^3=4^3=64 where digit 4 is equal to its cube units digit 4.
n^3=5^3=125 where digit 5 is equal to its cube units digit 5
HOWEVER, there is no actual answer i.e. whether to choose 4 or 5. So, it is insufficient.
If we take two statements together then we still don't find correct answer i.e. whether to choose 4,5 or 6. So, they are INSUFFICIENT together.
So the Answer is E. They together are also not sufficient.