Data Sufficiency

When the digits of two-digit, positive integer M are reversed, the result is the two-digit, positive integer N. If M > N, what is the value of M?

(1) The integer (M - N) has 12 unique factors.

(2) The integer (M - N) is a multiple of 9.

OAA

gmat_winter wrote:When the digits of two-digit, positive integer M are reversed, the result is the two-digit, positive integer N. If M > N, what is the value of M?

(1) The integer (M - N) has 12 unique factors.

(2) The integer (M - N) is a multiple of 9.

Let T = the TENS digit of M and U = the nonzero UNITS digit of M.
Then:
M = 10T + U.
Since the digits in N are reversed, N = 10U + T.

Resulting difference:
M-N = (10T + U) - (10U + T) = 9T - 9U = 9(T-U).

Since the greatest possible value of T-U = 9-1 = 8, M-N must be a MULTIPLE OF 9 NO GREATER THAN 9*8 = 72.
Thus, M-N = 9(T-U) must be equal to one of the following values:
9, 18, 27, 36, 45, 54, 63, 72.

Statement 1: The integer (M - N) has 12 unique factors.
Of the options for M-N, only 72 has 12 unique factors:
1*72
2*36
3*24
4*18
6*12
8*9.
Total factors = 12.

Since M-N = 9(T-U) = 72, we get:
9(T-U) = 72
T-U = 8, implying that T=9 and U=1.
Result:
M = 10T + U = 10(9) + 1 = 91.
SUFFICIENT.

Statement 2: The integer (M - N) is a multiple of 9.
No new information.
The prompt on its own implies that M-N is a multiple of 9.
INSUFFICIENT.

The correct answer is A.

making this Inference is difficult at first instance.

Since the greatest possible value of T-U = 9-1 = 8, M-N must be a MULTIPLE OF 9 NO GREATER THAN 9*8 = 72.
Thus, M-N = 9(T-U) must be equal to one of the following values:
9, 18, 27, 36, 45, 54, 63, 72.