The lunch lady has 4 pounds of lasagna. If she makes 1/3-pound servings using this amount of lasagna, how many servings can she make?

Divide 4 by 1/3, and you get the answer.

4/(1/3) = 12 servings

Your answer is 12 servings.
We can think about this in two ways:
Firstly, you can split up 4 pounds into 1/3-size pieces, and to find out how many you would need you would then have to divide 4 by 1/3:
4/1 ÷ 1/3
4/1 × 3/1 = 12/1 = 12 servings.
You can also think of it as how many 1/3-size pieces make up 1 pound, which is 3 because 3 × 1/3 = 1. Therefore you need 3 × 4 = 12 pieces.
I hope this helps! Let me know if you have any questions :)