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Source: — Data Sufficiency |
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by Stuart@KaplanGMAT » Tue May 13, 2008 1:47 pm
The general work formula is:

1/total time = 1/timeX + 1/timeY

We also know that total time = 48 and that all our variables must be non-negative (so we can ignore negative solutions).

So, we have 2 equations and 3 unknowns. 1 more equation and we can solve ANYTHING related to this system.

(1) timeX = 80

Another equation, can solve any question they could ask: sufficient.

(2) timeY = 120

Another equation, can solve any question they could ask: sufficient.

Each statement is sufficient on its own: choose (D).

I've said it before and I'll say it again - the # of equations/# of unknowns rule is THE most valuable rule for DS. Know the rule (and when to apply it) and know the exceptions (and when to apply them) and you'll avoid a lot of unnecessary calculation and get to answers much quicker.
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