To further clarify. The given question states “A company’s prices it’s product by using a mark up of 80% on variable production cost” , that means the company only uses variable cost for calculating their sales price. (sales = variable cost + profit ) therefore say variable cost = x that means profit = 80%(x) = .8x 15=x+1.8x 15/1.8=x=8.33
Now total cost of production = variable cost + fixed cost x + 50%(x) = 8.33+4.17 = 12.50