I curious,... I thought that in order to solve an equation (or in the case of a D.S. question - in order to know if one possibility is "SUFF"), that an equation must have only one variable.

In other words, if we are given the equation:

2a + 5b = 20

then we can't solve it without any other info. But if we try to simplify it down to finding "a" then we get:

2a = 20 - 5b or a = 10 - (5/2)b

If we try to plug in the equation again we get:

2[10 - (5/2)b] + 5b = 20 or 20 - 5b + 5b = 20 or

20 = 20

which leaves us with nothing.

But if a question has two equations, then we could use this plug-n-play approach to find the variables.


So, after trying to tackle the attached Kaplan D.S. question, I'm left wondering..... In both possibilities, it states that there is one equation, two variables but the first possibility states that we move to the second and then, all of a sudden, it's possible to solve one equation, two variables ??

What's going on here?