thephoenix wrote:A,B and C are deployed for a task, who is most efficient.

1) A and B together can do 70% of the work
2) B and C together can do 50% of the work

will post OA later

We know that together, A + B + C = 100% of the work

So, using THE most powerful DS tool, we realize that we have 1 equation and 3 unknowns; if we can get 2 more equations, we can answer any question about the system.

1) 1 more equation, nothing special: insufficient
2) 1 more equation, nothing special: insufficient

(Note that if the sum had been less than 50%, this would have been sufficient, since then A would have to do more than 50% of the work; but we can't assume that everyone does some of the work, so based on (2) alone it's possible that A does 50% and C does 50% and B does nothing, or that A does 50% and B does 50% and C does nothing - if the answer is stated as B then the author of the question is assuming that everyone does some work, which would never happen on the actual GMAT. Also, if the answer is B, I'd wonder if you reproduced the exact wording of the question.)

Together: 3 equations, 3 unknowns - sufficient!

If you approached the question this way, it literally takes 15 seconds to solve. The more often you look for situations in which to apply the n linear equations rule, the more time you'll save in DS.

As an aside, if this had been problem solving and we actually cared what the answer was:

A + B + C = 100
A + B = 70

subtracting the second equation from the first: C = 30

B + C = 50

Subbing in C = 30 to the third equation: B = 20

If C=30 and B = 20, A = 50.