We have two functions.
One determines the cost of the materials, [tex]f(x)=\frac{4}5x+4[/tex].
The other determines the selling price, [tex]g(x)=4x+5[/tex].
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).
[tex]f(15)=\frac{4}5\times15+4[/tex]
[tex]f(15)=12+4[/tex]
[tex]f(15)=16[/tex]
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).
[tex]g(17)=4\times16+5[/tex]
[tex]g(17)=64+5[/tex]
[tex]\boxed{g(17)=69}[/tex]
