Asuming that the 'hardware' is not a one time cost, that'll also be included in the expression, so it is now along the lines of 30 (f(x) + 3.75).
We also are told that each bead costs .5, we dont need to know the number as that is factored in during the order, and assuming that she buys the same amount of beads for each necklace, we'll be using 0.5b in the equation.
Now the expression is 30 (0.5b + 3.75) to work out the cost of 30 necklaces.
