Hi ziyuenlau,
This DS question doesn't actually require that much 'math' - but you do have to keep track of the information that you're given to get the correct answer.
We're told that Machine X and Machine Y each have a constant rate of production (Machine X's rate is 400 bolts/hour) and that the Machines run simultaneously. We're asked how many bolts are produced by the 2 machines per hour.
1) Machine X takes twice as long to produce 400 bolts as it does for machines X and Y, working together, to produce the same number of bolts.
We already know that Machine X produces 400 bolts in 1 hour. Fact 1 tells us that 1 hour is TWICE as long as the time it takes the two machines to produce those same 400 bolts. Thus, the two machines (working together) can produce 400 bolts in 1/2 hour. We know that Machine X would produce 200 of those bolts in 1/2 hour, so the other 200 bolts must be produced by Machine Y during that time. We now know Machine Y's rate (it's 400 bolts/hour).
Fact 1 is SUFFICIENT.
2) Machines X and Y produce bolts at the same rate.
Since we already know that Machine X produces 400 bolts/hour, we now know that Machine Y also produces 400 bolts/hour.
Fact 2 is SUFFICIENT.
Final Answer: D
GMAT assassins aren't born, they're made,
Rich
