Linear Programming Blend problem: I need to know how they got their decision variables, please explain it to me like I'm 5 years old!

A Blend Example A petroleum company produces three grades of motor oil-super, premium, and extrafrom three components. The company wants to determine the optimal mix of the three components in each grade of motor oil that will maximize profit. The maximum quantities available of each component and their cost per barrel are as follows: Maximum Barrels Available/Day Component Cost/Barrel 1 2 3 4,500 2,700 3,500 $12 10 14 To ensure the appropriate blend, each grade has certain general specifications. Each grade must have a minimum amount of component 1 plus a combination of other components, as follows: Component Specifications Selling Price/Barrel Grade Super $23 Premium 20 At least 50% of 1 Not more than 30% of 2 At least 40% of 1 Not more than 25% of 3 At least 60% of 1 At least 10% of 2 Extra 18 The company wants to produce at least 3,000 barrels of each grade of motor oil. Decision Variables The decision variables for this problem must specify the quantity of each of the three compo- nents used in each grade of motor oil. This requires nine decision variables, as follows: Xij = barrels of component i used in motor oil grade j per day, where i = 1, 2, 3 and j = s (super), p (premium), e (extra). For example, the amount of component 1 in super motor oil is X1s. The total amount of each grade of motor oil will be super: Xis + x2 + x3x premium: Xip + X2p + X3p extra: Xle + x2 + x3e