Problem 10 (2002 AMC 8)

Juan organizes the stamps in his collection by country and by the decade in which they were issued. The prices he paid for them at a stamp shop were: Brazil and France, $6$ cents each, Peru $4$ cents each, and Spain $5$ cents each. (Brazil and Peru are South American countries and France and Spain are in Europe.)

Number of Stamps by Decade Country | '50s | '60s | '70s | '80s Brazil | $4$ | $7$ | $12$ | $8$ France | $8$ | $4$ | $12$ | $15$ Peru | $6$ | $4$ | $6$ | $10$ Spain | $3$ | $9$ | $13$ | $9$

The average price of his '70s stamps is closest to