I'd like to understand how to do this problem

errapin Rickshaw is currently operating at 80% of capacity and is currently purchasing a part used in its manufacturing operations for $25 unit. The unit cost for Terrapin Rickshaw to make the part is $30, which includes $3 of fixed costs. If 20,000 units of the part are normally purchased each year but could be manufactured using unused capacity, what would be the amount of differential cost increase or decrease for making the part rather than purchasing it?