The easiest way to solve this would be by setting up a table of sorts.
Let us assume that initially A had a chocolates, B had b chocolates, C had c chocolates and D had d chocolates.
Given: \(a = b+80\)
We need to find: \(c-d\)
Here's a table I created with the information we've been given.
Attachment:
Table .jpg [ 33.25 KiB | Viewed 19623 times ]
So now let us look at the other things we've been given. Finally, each girl has the same number of chocolates. So that means:
\(\frac{2a}{3}= \frac{3}{4} (b + \frac{a}{3}) = \frac{4}{5} (c + \frac{b}{4} + \frac{a}{12}) = d + \frac{1}{5} ( c + \frac{b}{4} + {a}{12})\)
So now take this one at a time.
\(\frac{2a}{3}= \frac{3}{4} (b + \frac{a}{3})\)
But you know that a = b + 80. So substitute that into this equation.
\(\frac{2b + 160}{3}= \frac{3}{4} (b + \frac{b + 80}{3})\)
Simplifying this we get:
\(\frac {3b+a}{4} = \frac{4c}{5} + \frac{b}{5} + \frac{a}{15}\)
Substituting a = 80 and b = 100 in this, we get c = 110
Now solve for the last part of the equation:
\(\frac{4}{5} (c + \frac{b}{4} + \frac{a}{12}) = d + \frac{1}{5} ( c + \frac{b}{4} + \frac{a}{12})\)
Taking everything except d to the left hand side we get:
\(\frac{3}{5} (c + \frac{b}{4} + \frac{a}{12} = d\)
Now substituting values of a, b and c into the equation as 180, 100 and 110 respectively we get the value of d as:
\(d = 90\)
Thus \(c - d = 110 - 90 = 20.\)
Hence the answer is A.
P.S - This problem seems way too lengthy.