At least how many grams of pure bronze must be...

swerve wrote:At least how many grams of pure bronze must be alloyed with 387 grams of pure copper to create an alloy that is no more than 90% copper?

A. 13
B. 43
C. 103
D. 348
E. 430

The OA is B.

Please, can any expert explain this PS question for me? I tried the following,

Can I say that
387 + x grams = y grams,
and
387 = 0.9y then y = 430.

Finally,
387 + x = 430
x = 430 - 387 = 43.

I need your help. Thanks.

A bit of logic eliminates most of the answer choices. Clearly, D and E would yield a mixture with much less than 90% copper. And if we estimate, we can see the same is true of C - if a mixture were 400 grams copper and you added 100 grams of bronze, the total mixture of 500 grams would be only 80% copper. C is out.

Now you only have to test one of the two remaining options. Say you tried A. If you added 13 grams of bronze, the total mixture would be 387 + 13 = 400 grams. If it were 90% copper, it would have had .9 * 400 = 360 grams copper. If there's actually 387 grams, there's clearly more than 90% copper. A is out.

You're left with B