Take the least value which will suffice the answer statements.
Keep in mind whether the numbers are natural or fractional or whole etc
Example
(1) when x is divided by 3, the remainder is 2.
Least value that x can take 2,5,8,11 an so on.
(2) when y is divided by 6, the remainder is 5. '
Least value that y can take 5,11,17,23 an so on.
Now deduce the values (or maybe a combination of values and try to see if you get the answer)
Lets consider statement (1) alone first
Not sufficient, since information on Y is missing
Lets consider statement (2) alone first
Not sufficient, since information on X is missing
Consider together
Deduce
assume X = 2 and Y = 5
2+5-1 = 6 (divisible by 3)
assume X = 2 and Y = 11
2+11-1 = 12 (divisible by 3)
You can also put some more values to test the same.
Ans will be both statements are together sufficient to answer the Question.
NOTE: You can also prove this by method of mathematical induction . (Just google to learn more about this )
based on the time you get in GMAT, you can take appropriate route.
DK....
