Hi M7MBA,

We're told that of the 2,500 tons of ore mined daily at a quarry, 0.4% results in a certain pure metal. We're asked for the number of days of mining that are needed for the total amount of pure metal produced at the quarry to be equal to the daily amount of ore mined. This question can be solved Algebraically; you can also use the 'spread' of the answer choices and a little logic to solve it without doing much math at all.

If you wanted to do the Algebra, you could work through the following steps:

2500(.04) = 10 tons of pure metal mined each day.

To get 2500 tons of pure metal:
2500 = 10X where X is the number of days of mining
2500/10 = X
X = 250 days

From a pattern/logic standpoint though, we know that if 1% of what we mined each day was pure metal, then it would take 100 days for the total pure metal to equal the 2500 tons of mined material from each day. Since we're actually mining 0.4% of pure metal each day - and 0.4% is a little LESS than HALF a percent, then we will need a little MORE than DOUBLE the amount of time to hit that same total. Thus, we're looking for an answer that's a little greater than 200 days - and there's only one answer that matches...

Final Answer: C

GMAT assassins aren't born, they're made,
Rich