It's not as simple as switching numbers because they sometimes ask questions from a different angle or with a subtle trick where you have to really think a little and if you plugged in numbers to a shortcut or formula you would get the answer wrong.

E.g. they tell you perimeter of an iscoeles right triangle is 16 + 16√2. What is the hypotenuse?

You may want to plug in formula for iscoseles right triangle 2x + x√2 = Perimeter.
So 2x + x√2 = 16 + 16√2.
Therefore you'd say x = 8 and hypotenuse is = 8√2.

But this is wrong.

The answer is hypotenuse is 16. The legs are 8√2 each and the hypotenuse is 8√2√2 = 16.

So the difficulty is really in understanding the subtleties in the questions being asked. Here it is not enough to know the formula for right iscoceles triangles and be able to switch numbers; in fact the question tries to bait you into doing just that. Here you are also required to understand the properties of roots by recognizing that because 16 is actually 8√2√2, x is in fact 8√2 and not 8.

This is a classic GMAT trick/trap question you'll see at the higher end of questions... Deceptively easy but with a wicked twist.