We have two functions.
One determines the cost of the materials, \(f(x)=\frac{4}5x+4\).
The other determines the selling price, \(g(x)=4x+5\).
Now one thing to notice is that in the first function, x is the number of biscuits.
In the second function, however, x is the cost of the materials, as indicated by the price of the biscuits being g(f(x)).
So, let's find the cost of the materials for making 15 biscuits.
Use 15 for x in our function f(x).
\(f(15)=\frac{4}5\times15+4\)
\(f(15)=12+4\)
\(f(15)=16\)
Now that we know f(x) (the cost of the materials), we can find g(f(x)).
Use 16 for x in our function g(x).
\(g(17)=4\times16+5\)
\(g(17)=64+5\)
\(\boxed{g(17)=69}\)
