For positive integers x and y, x=19!+y. Is x a prime number?

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have 2 variables and 1 equation, D is most likely to be the answer. So, we should consider each condition on its own first.

Condition 1)
The possible values of \(y\) are 38, 39, 40.
If \(y=38=2*19\), we have \(19!+38 = 19! + 2*19 = 18!*19 + 2*19 = (18!+2)*19\), which is not a prime number.
If \(y=39=3*13\), we have \(19!+39 = 1*2*3*4*...*19 + 3*13 = 3*(2*4*5*...*19) + 3*13 = 3*((2*4*5*...*19)+13)\), which is not a prime number.
If \(y=40=2*20\), we have \(19!+39 = 1*2*3*4*...*19 + 2*20 = 2*(3*4*5*...*19) + 2*20 = 2*((3*4*5*...*19)+20)\), which is not a prime number.

Since 'no' is also a unique answer by CMT (Common Mistake Type) 1, condition 1) is sufficient.

Condition 2)
If \(y=2*k\), we have \(19!+2k = 1*2*3*4*...*19 + 2k = 2*(3*4*5*...*19) + 2k = 2*((3*4*5*...*19)+k)\), which is not a prime number.

Since 'no' is also a unique answer by CMT (Common Mistake Type) 1, condition 2) is sufficient.

Therefore, D is the answer.

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A,B,C or E.