Answer to Question #70595 in Microeconomics for Johnathan McGuire
Your budget constraint for the two goods A and B is 12A + 4B = I, where I is your income. You are currently consuming more than 27 units of B. In order to get 3 more units of A, how many units of B would you have to give up?
at first, let's express A in terms of I and B:
A = I / 12 - B / 3
now, given the same income (that is, I is constant) suppose we decrease the level of consumption of good B from B1 to B2. How many additional units of A (dA = A2-A1) can we get within the same budget constraint?