C(m) = $5.00*W(m)*lambda + $6.00*m.
Choose integer m to minimize C(m) given m > 3. I find the minimizing m = 4 with cost $31.64.
The formula for W(m) is too much to write out here without LaTeX. It is shown here with rho = lambda/m*mu.
For (ii) I find the minimizing m = 7 with cost $45.13.