Step 1 :The question is asking for the inverse function of $P(n)$, which is denoted as $P^{-1}(x)$. The inverse function is used to find the original input given the output of a function. In this case, $P^{-1}(x)$ would represent the amount of vitamins (in grams) for a price of $x$ dollars.
Step 2 :The original function is $P(n) = 0.8n + 5.2$. If we replace $P(n)$ with $x$ and $n$ with $P^{-1}(x)$, we get $x = 0.8P^{-1}(x) + 5.2$.
Step 3 :We can solve this equation for $P^{-1}(x)$ to find the inverse function. The inverse function $P^{-1}(x)$ is $1.25x - 6.5$.
Step 4 :Finally, we can substitute $x = 10.4$ into the inverse function to find $P^{-1}(10.4)$. The result is $P^{-1}(10.4) = \boxed{6.5}$ grams.