Daisy received a tape recorder as a birthday gift and is not able to return it. Her utility function is U(x, y, z) = x + z1/2 f(y), where z is the number of tapes she buys, y is the number of tape recorders she has, and x is the amount of money she has left to spend. f(y) = 0 if y < 1 and f(y) = 8 if y is 1 or greater. The price of tapes is $1 and she can easily afford to buy dozens of tapes. How many tapes will she buy?