Who among the following has the highest purchasing power?
a. Stacy, whose monthly income and cost of living are $5500 and $4200, respectively.
b. James, whose monthly income and cost of living are $3000 and $1500, respectively.
c. Martha, whose monthly income and cost of living are $4000 and $2700, respectively.
d. Peter, whose monthly income and cost of living are $5000 and $3800, respectively.
Answer: b. James, whose monthly income and cost of living are $3000 and $1500, respectively.