talaangoshtari wrote:Two alloys of gold, P and Q, prepared by mixing copper and gold in the ratio (2 : 7) and (11 : 7) respectively. If equal quantities of the alloys are melted to form a third alloy R, the ratio of copper and gold in R will be:
A.4:9
B.7:5
C.5:7
D.9:5
E.4:7
I prefer Mitch's approach, but here's another approach that works:
This can be viewed as a "weighted" averages where each alloy gets the same weight. By the way, we have a free video on weighted averages:
https://www.gmatprepnow.com/module/gmat- ... ics?id=805
Alloy P: copper and gold in 2:7 ratio
This means that 2/9 of alloy P is copper
Or we can say that
4/18 of alloy P is copper
Alloy Q: copper and gold in 11:7 ratio
This means that
11/18 of alloy Q is copper
Since we're adding EQUAL QUANTITIES of the two alloys, the resulting alloy (R) will have a mixture that is the same as AVERAGE of the two alloys.
The AVERAGE of
4/18 and
11/18 = 7.5/18
So, in 18 pounds of alloy R, copper comprises
7.5 pounds, which means gold comprises the other
10.5 pounds
So, the ratio of copper and gold in alloy R =
7.5 :
10.5
This does not appear as one of the answer choices, so we'll need to examine EQUIVALENT ratios.
7.5 :
10.5 = 15 : 21 = [spoiler]5 : 7[/spoiler]
Answer:
C
