Assume each part of chocolate is 100 (grams or whatever). Then quantity A contains \(\frac{3*80+0}{400}*100 = 60%\) cocoa, while quantity B contains \(\frac{2*50+80}{300}*100 = 60%\) cocoa.

Answer C

I've never solved this correctly in practise, however, I just saw this today and my solution to get C is below:

QTY A = 400%, adding all the cocoa mixtures = 240%. So 240/400 = 3/5
QTY B = 300%, adding all the cocoa = 180%. So 180/300 = 3/5

Hope this helps someone who struggled with this as much as I did.

Or without making any assumption, and just work with the portion given in the question:
Quantity A : 3 parts Silky Dark Chocolate which is 80% cocoa, and 1 part Smooth White Chocolate with 0% cocoa, all together our chocolate is combined of 4 parts as you can see. So, we have: (3/4 x 0.8) + (1/4 x 0.0)= 0.6 or 60%

Quantity B : 2 parts Rich Milk Chocolate which is 50% cocoa, and 1 part Silky Dark Chocolate with 80% cocoa. Here, the chocolate is combined of 3 parts. So, we have: ( 2/3 x 0.5 ) + ( 1/3 x 0.8 ) = 0.6 or 60%

As we can see quantity A is equal to quantity B.

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