if a is a positive integer, and if the unit's digit of a^2 is 9 and the unit's digit of (a+1)^2 is 4, what is the unit's digit of (a+2)^2 ?
a. 1
b. 3
c. 5
d. 7
e. 9
answer is A
unit digit
This topic has expert replies
-
- Legendary Member
- Posts: 752
- Joined: Sun May 17, 2009 11:04 pm
- Location: Tokyo
- Thanked: 81 times
- GMAT Score:680
unit digit of a must be 3 or 7 because unit digit of a^2 is 9.
unit digit of (a+1) must be 2 or 8 because (a+2)^2 has the unit digit 4
from this, unit digit of a must be 7
7+2=9
9^2 has the unit digit 1
so A
unit digit of (a+1) must be 2 or 8 because (a+2)^2 has the unit digit 4
from this, unit digit of a must be 7
7+2=9
9^2 has the unit digit 1
so A
The powers of two are bloody impolite!!
This problem was printed in OG Quant Review p80 #142. My question is, am I the only person confused by the wording of this problem? At first I really couldn't figure out what the heck they were asking.
They should have said something like the (unit digit of a)^2. the [(unit digit of a)+1]^2, etc. Basically I'm saying that they messed up the parenthesis and really threw me off.
They should have said something like the (unit digit of a)^2. the [(unit digit of a)+1]^2, etc. Basically I'm saying that they messed up the parenthesis and really threw me off.