Answer :
I think that it is B because I'm pretty sure it has to be log of 50,000. But I forgot how to get the rest of it but B is the only one with log of 50k.
f(x) = 50,000 (0.8)^x
⇒ y = 50,000 (0.8)^x
⇒ log y = log 50,000 + x log 0.8
⇒ x log 0.8 = log y - log 50,000 = log (y/50,000)
⇒ x = log(y/50,000) / log 0.8
⇒ x = log0.8(y/50,000)
⇒ f^-1(x)=log0.8(x/50,000)
Answer is D
Hope it helps!
⇒ y = 50,000 (0.8)^x
⇒ log y = log 50,000 + x log 0.8
⇒ x log 0.8 = log y - log 50,000 = log (y/50,000)
⇒ x = log(y/50,000) / log 0.8
⇒ x = log0.8(y/50,000)
⇒ f^-1(x)=log0.8(x/50,000)
Answer is D
Hope it helps!