we don't need the complicated data to calculate the the number of current customers who order coffee(I will just assume there are X customers ordering free coffee currently). We don't need to know how many exactly the X is we can make the decision that B-coffee can help AA get more profit. 

here is the calculation:

For B-coffee: $4/person, 10% profit margin(50%*20%), X current customers ordering coffee, with B-coffee the number will go down 25%. profit=X*(1-75%)*4*10%=0.3X

For current coffee:$3/person, 20% profit margin, X current customers ordering coffee, with current-coffee the number will go down 70%. profit=X*(1-70%)*4*10%=0.18X, which is obviously smaller than 0.3X since X must be positive.

All those complicated data is just for us to calculate X, which doesn't need to know at all for solving the problem.