5.33. Gina lives in Chicago and very much enjoys traveling by air to see her mother in Italy. On the accompanying graph, x denotes her number of round trips to Italy each year. The composite good y measures her annual consumption of other goods; the price of the composite good is py, which is constant in this problem. Several indifference curves from her preference map are drawn, with levels of utility U1 $ U2 $ U3 $ U4 $ U5. If she spends all her income on the composite good, she can purchase y* units, as shown in the graph. When the initial price of air travel is $1,000, she can purchase as many as 18 round trips if she spends all her income on air travel to Italy?