VeritasPrepKarishma wrote:
You can easily use the scale method here. The scale method is explained here:
tough-ds-105651.html#p828579Focus on any one of the two elements say Gold.
First bar has 2/5 gold. Second bar has 3/10 gold and mixture has 5/16 gold.
Make the fractions comparable for easy calculation i.e. give them the same denominator. LCM of 5, 10 and 16 is 80.
First bar has 32/80 gold. Second bar has 24/80 gold and mixture has 25/80 gold.
Attachment:
Ques3.jpg
So first bar weight:second bar weight should be in the ratio 1:7. Out of 8 total kgs, first bar must have been 1 kg.
Responding to a pm:
Think of what the formula is:
w1/w2 = (C2 - Cavg)/(Cavg - C1)
First bar: C1 = 32/80, w1 = weight of first bar
Second bar: C2 = 24/80, w2 = weight of second bar
Cavg = 25/80
Simply plug these values in the formula.
w1/w2 = (C2 - Cavg)/(Cavg - C1) = (24/80 - 25/80)/(25/80 - 32/80) = 1/7
w1:w2 = 1:7
You don't need to worry about anything else.
When using the scale method, we flip the ratio because we calculate (Cavg - C1) first and (C2 - Cavg) later. This is opposite to the way it is in the formula so we flip the ratios.
Above, when I made the scale, I put the second bar first and the first bar later. The reason was that it is more intuitive that way on the number line since C2 = 24/80 is smaller than C1 = 32/80. Since I was finding Cavg - C2 first and C1 - Cavg later, I didn't need to flip the ratios.
My advice would be to simply identify one element as Element1, another as Element2 and figure out C1, w1 and C2, w2 for the 2 of them and simply plug in the formula. There will be no confusion in that case.
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Karishma
Veritas Prep GMAT Instructor
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