12843-3955=____
请回答:
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已知函数f(x)=2x+3,f-1(x)是f(x)的反函数,若mn=16(m,n∈R+),则f-1(m)+f-1(n)的值为 ( )
—May I speak to Mr. Smith?
—Sorry, he _______ Australia. But he _______ in two days.