-
gmatusa2010
- Master | Next Rank: 500 Posts
- Posts: 134
- Joined: Sun Jul 25, 2010 5:22 am
- Thanked: 1 times
- Followed by:2 members
Problem is asking for units digit of 9^(2a + 1 + b).gmatusa2010 wrote:If a and b are positive integers, what is the remainder when
9^(2a+1+b) is divided by 10?
(1) a = 3
(2) b is odd.
Now if the power of 9 is odd => Unit's digit = 9
And if the power of 9 is even => Unit's digit = 1
Thus we can determine the unit's digit of 9^(2a + 1 + b), if we know whether (2a + 1 + b) is odd or even.
Statement 1: a = 3
(2a + 1 + b) = (b + 7)
We don't whether b is odd or even.
Not sufficient
Statement 2: b is odd
(2a + 1 + b) = (Even + 1 + Odd) = Even
Sufficient
The correct answer is B.
