I know the correct answer but I need the solution. How do I calculate this problem?

Question 4 1/1 pts The ABC Widget Integrated Supply Chain Consider the following scenario for the ABC Widget Integrated Supply Chain: a multi- tiered supply chain consisting of a Wholesaler (W), a Retailer (R), and a Manufacturer (M). The Wholesaler supplies a product to a Retailer (R), which, in turn, sells the product to end Customers (C). C pays R$6.50 per unit. R pays W $4.00 per unit. W pays the Manufacturer (M) $1.40 per unit. The costs to Mare $.75/unit. Note that ABC Widget sales are seasonal and unsold widgets have no retail value after the end of the season. The optimal service level for the integrated supply chain is: 38% 62% 79% 88% 24%