Assume that the domestic demand for mp3 players is given by P = 80 - Q. The supply of domestic producers is given by P = 10 + Q, and the international supply by P = 15. If this is an open economy then the number of quantities of
import of mp3 for this economy is:
a) 50
b) 60 c) 40 d) 20
答案是选择B 为什么呢