sherxon
How much water did the swimming pool have initially, if 400 cubic meters of water was left after the emptying pipe had been open for an hour?
1) 250 cubic meters of water was left in the swimming pool after the emptying pipe had been open for four hours
2) 50 cubic meters of water per hour can flow through the emptying pipe
If we assume the 'emptying pipe' empties the pool continuously, at a constant rate, then the answer is D; Statement 1 tells us the water dropped by 150 cubic meters in 3 hours, or 50 cubic meters each hour, so the pool contained 450 cubic meters an hour ago, and Statement 2 tells us the same thing. So under that assumption, the answer is D.
But the question doesn't tell us the pipe empties the pool at a constant rate, and if you don't know that, you can't do anything here. I'd bet the source says the "OA" is D here, but the answer is E (either way it's not a fair question, because it's not clear what you're meant to assume). Where is the question from?