If Robert types a 120 page manuscript at a rate of 50 words per minute, how long will it take him to type the manuscript?
(1) There are 240 words per page in the manuscript
(2) If the manuscript were 10 pages longer, it would take Robert 48 minutes longer to type it.
(1) 120*240/50 .... suffices.
(2) Rob types at 10 pages every 48 mins. Which is 50 words/min. This can give us the required time IF the manuscript was uniformly worded. However this statement doesn't assure us that it is indeed the case. Thus doesn't suffice.
This gives option A as the answer.
Who says elephants can't dance?