This problem involves creating a linear programming model to minimize the cost of soybean meal and oats while satisfying the nutritional requirements for protein, fat, and mineral ash.

Step 1: Define Variables

Let:

• $x$ be the number of sacks of soybean meal.
• $y$ be the number of sacks of oats.

Step 2: Objective Function

The cost function (which we want to minimize) is: $\text{Minimize:} \ C = 21x + 7y$ This is the total cost, where each sack of soybean meal costs $21 and each sack of oats costs $7.

Step 3: Constraints

We have three constraints based on the nutritional content (protein, fat, and mineral ash requirements):

1. Protein: Each sack of soybean contains 70 lb of protein, and each sack of oats contains 21 lb of protein. We need at least 168 lb of protein: $70x + 21y \geq 168$
2. Fat: Each sack of soybean contains 9 lb of fat, and each sack of oats contains 7 lb of fat. We need at least 27 lb of fat: $9x + 7y \geq 27$
3. Mineral Ash: Each sack of soybean contains 7 lb of mineral ash, and each sack of oats contains 1 lb of mineral ash. We need at least 14 lb of mineral ash: $7x + y \geq 14$

Additionally, the number of sacks $x$ and $y$ must be non-negative: $x \geq 0, \quad y \geq 0$

Step 4: Solve the Linear Program

We now solve this linear program:

• Objective: Minimize $21x + 7y$
• Subject to: 70x + 21y &\geq 168 \\ 9x + 7y &\geq 27 \\ 7x + y &\geq 14 \\ x &\geq 0 \\ y &\geq 0 \end{aligned}$$

Step 5: Solve Graphically or via Simplex Method

To find the optimal values of $x$ and $y$, we can either graph the inequalities or use a method such as the Simplex algorithm. Graphically, the intersection points of the constraint lines with the axes will be evaluated, and the one that minimizes the cost function will give the optimal solution.

Would you like me to perform the solution using one of these methods?