Bob consumes two goods, x and y. His utility function is given by u(x, y) = xy. Bob's income is $72 per day. The price of one unit of y is always $1 per unit. Yesterday, the price of one unit of good x was $9. Today, the price of one unit of good z is $4. Calculate the income, substitution and total effects for good of this price decrease of good r. In order to do so, compute yesterday's optimal bundle and today's optimal bundle for Bob, as well as the bundle that is optimal given the appropriate "hypothetical" budget line.