Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and independent equations ensures a solution.
If a triangle has one side of 3 cm and another side of 4 cm, how long is the third side?
(1) The triangle is a right triangle.
(2) The third side is the longest side.
Modify the original condition and the question. Suppose the length of the third side as x, it becomes 4-3<x<4+3, 1<x<7. Then, there is 1 variable(x), which should match with the number of equations. So you need 1 equation. For 1) 1 equation, for 2) 1 equation, which is likely to make D the answer.
For 1), you cannot certain that x is the longest side. So, 3^2+4^2=x^2 or 3^2+x^2=4^2, which is not unique and not sufficient.
For 2), even if it is the longest side, you cannot figure out the side in a unique way, which is not sufficient.
When 1) & 2), x=5, which is unique and sufficient.
Therefore, the answer is C.
For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.