For these type of problems, the best way is substitution.
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....
