$-1\mathrm{.}13$ Refer to Problem $1\mathrm{.}12.$ The pay will be $$8$ per hour for employees. Charles Lackey can also improve the yield by purchasing a new blender. The new blender will mean an increase in his investment. This added investment has a cost of $$100$ per month, but he will achieve the same output $($an increase to $1,875)$ as the change in labor$-$hours. Which is the better decision?

a$)$ Show the productivity change, in loaves per dollar, with an increase in labor cost $($from $640$ to $800$ hours$).$

b$)$ Show the new productivity, in loaves per dollar, with only an increase in investment $($ $$100$ per month more$).$

c$)$ Show the percent productivity change for labor and investment.