A mall has three department stores, JC Penney, Macy's, and Sears. A survey of 2,118 people exiting the mall found that 433 made a purchase at JC Penney, 412 made a purchase at Macy's, 638 made a purchase at Sears, 109 made a purchase at JC Penney and Macy's, 167 made a purchase at JC Penney and Sears, 213 of them made a purchase at Macy's and Sears, and 60 of them made a purchase at all three stores.
a) How many people made a purchase at Sears but did not make a purchase at JC Penney?
b) How many people made a purchase at exactly one of the three stores?