Question 1200730
**a) Find the Marginal Cost**

1. **Find the derivative of the cost function:**
   * C'(u) = d/du (-0.02u² + 2.33u + 11.61) 
   * C'(u) = -0.04u + 2.33

2. **Evaluate the derivative at u = 21 (production level of 21 thousand pairs):**
   * C'(21) = -0.04 * 21 + 2.33 
   * C'(21) = 1.49

**Therefore, the marginal cost at a production level of 21 thousand pairs of glasses is $1.49 thousand per thousand pairs (or $1490 per thousand pairs).**

**b) Find the Average Cost**

1. **Calculate the total cost at u = 21:**
   * C(21) = -0.02 * (21)² + 2.33 * 21 + 11.61 
   * C(21) = -8.82 + 48.93 + 11.61 
   * C(21) = 51.72 thousand dollars

2. **Calculate the average cost:**
   * Average Cost = Total Cost / Number of Units
   * Average Cost = 51.72 thousand dollars / 21 thousand pairs
   * Average Cost = 2.46 dollars per pair

**Therefore, the average cost at a production level of 21 thousand pairs of glasses is $2.46 per pair.**