Practice GMAT Data Sufficiency Question
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If w, x, y, and z are the digits of the fourdigit number N, a positive integer, what is the remainder when N is divided by 9?
 w + x + y + z = 13
 N + 5 is divisible by 9
Correct Answer: D
 In order for a number, n, to be divisible by 9, its digits must add to nine. Likewise, the remainder of the sum of the digits of n divided by 9 is the remainder when n is divided by 9. In other words:
 To see this, consider a few examples:
Let N = 901
901/9 = 100 + (R = 1)
(9+0+1)/9 = 10/9 = 1 + (R = 1)
Let N = 85
85/9 = 9 + (R = 4)
(8+5)/9 = 1 + (R = 4)
Let N = 66
66/9 = 7 + (R = 3)
(6+6)/9 = 1 + (R = 3)
Let N = 8956
8956/9 = 995 + (R = 1)
(8+9+5+6)/9 = 28/9 = 3 + (R = 1) 
Evaluate Statement (1) alone.
 Based upon what was shown above, since the sum of the digits of N is always 13, we know that remainder of N/9 will always be the remainder of 13/9, which is R=4.
 In case this is hard to believe, consider the following examples:
4540/9 = 504 + (R = 4)
(4+5+4+0)/9 = 13/9 = 1 + (R = 4)
1390/9 = 154 + (R = 4)
(1+3+9+0)/9 = 13/9 = 1 + (R = 4)
7231/9 = 803 + (R = 4)
(7+2+3+1)/9 = 13/9 = 1 + (R = 4)
1192/9 = 132 + (R = 4)
(1+1+9+2)/9 = 13/9 = 1 + (R = 4)  Statement (1) is SUFFICIENT.

Evaluate Statement (2) alone.
 If adding 5 to a number makes it divisible by 9, there are 95=4 left over from the last clean division. In other words, N/9 will have a remainder of 4.
 To help see this, consider the following examples:
Let N = 4
N+5=9 is divisible by 9 and N/9 > R = 4
Let N = 13
N+5=18 is divisible by 9 and N/9 > R = 4
Let N = 724
N+5=729 is divisible by 9 and N/9 > R = 4
Let N = 418
N+5=423 is divisible by 9 and N/9 > R = 4  Since N + 5 is divisible by 9, we know that the remainder of N/9 will always be 4. Statement (2) is SUFFICIENT.
 Since Statement (1) alone is SUFFICIENT and Statement (2) alone is SUFFICIENT, answer D is correct.
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