Question 1170861
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You have received, to this point, two formal algebraic solutions.  One uses a typical approach and gets to an answer relatively quickly.  The other uses a very roundabout approach, eventually reaching an answer.<br>
If a formal algebraic solution method is not required, here is the fastest and easiest way to get the answer.<br>
You are mixing 10-kt and 18-kt gold to get 16-kt gold.
Consider the three numbers on a number line.  16 is 3/4 of the way from 10 to 18.
That means 3/4 of the mixture needs to be the 18-kt gold.<br>
ANSWER: 3/4 of 200g, or 150g, is the 18-kt gold; the other 50g is the 10-kt gold.<br>