Ametallurgist needs to make 12.4 lb. of an 10) alloy containing 50% gold. he is going to melt and combine one metal that is 60% gold with another metal that is 40% gold. how much of each should he use?

Now, let's say, we add "x" lbs of the 60% gold alloy, so..  how much gold is in it?  well, is just 60%, so (60/100) * x, or 0.6x.

likewise, if we use "y" lbs of the 40% alloy, how much gold is in it?  well, 40% of y, or (40/100) * y, or 0.4y.

now, whatever "x" and "y" are, their sum must be 12.4 lbs.

we also know that the gold amount in each added up, must equal that of the 50% resulting alloy.





how much of the 40% alloy?  well, y = 12.4 - x.

The first one, i believe, would be correct. all the others have one thing in common, and that's that they have repeating numbers. in the second one, x and y have repeating 6's and 3's, and in the third, x and y have repeating 6's as well. in the last one, there are repeating 3's. the first one has no repeating numbers.