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.
GMAT Prep (Working Rates) Pract 2
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