LCC

As I understood from the chapter, By spending money on Design and Research the Manufacturing cost can be reduced.

In chapter 3, example 1.

c) in the answers its said:
(60000+20000+30000)/50000=2.20
Therefore, the max manufacturing cost per unit would have to fall from $6.00 to ($7.00 – $2.20) = $4.80 per unit

but my question is: only $20,000 that is going to be spent extra on Design can reduce the manufacturing cost, not 60000 and 30000 that is already allocated to manufacturing costs per unit and resulted in cost of $6.00 per unit. I dont get the logic, sir. Why we use the money that is already going to be spent on reducing the Cost per unit if its not meant to be reducing it?