If the product of the digits of a two-digit number x is 12, what is the value of x?
(1) The units digit of x is a prime number.
(2) x is even.
If we look at the original condition, we get x=26,34,43,62. There is 1 variable (x), and in order to match the number of variables to the number of equations, we need 1 more equation. Since the condition 1) and the condition 2) each has 1 equation, there is high chance that D is the answer.
For the condition 1), since x=43, 62, the answer is not unique and the condition is not sufficient.
For the condition 2), since x=26, 62, the answer is not unique and the condition is not sufficient.
Using both the condition 1) and the condition 2), we get x=62. The answer is unique and the condition is sufficient. Therefore, the correct answer is C.
l For cases where we need 2 more equations, such as original conditions with “2 variables”, or “3 variables and 1 equation”, or “4 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 70% chance that C is the answer, while E has 25% chance. These two are the majority. In case of common mistake type 3,4, the answer may be from A, B or D but there is only 5% chance. Since C is most likely to be the answer using 1) and 2) separately according to DS definition (It saves us time). Obviously there may be cases where the answer is A, B, D or E.