Which is a complete sentence? Egg yolks contain far more protein and fat than egg
whites do.
Multiplying fractions and decimals isn't hard, I can
show you how.

Egg yolks contain far more protein and fat than egg whites do.

Explanation:

A complete sentence has to have a subject, a verb, and a complete thought. That last part is going to be important as we rule out the second sentence.

The phrases "Multiplying fractions and decimals isn't hard" and "I can show you how" are two completely different thoughts. You can't just add a comma after a complete thought and then stick a different one after it. You have to add a conjunction or ";" symbol.

So, we can say that the second sentence is not a complete sentence since it has two different complete thoughts that aren't joined together properly.

The first sentence has a subject (egg yolks), a verb (contain), and it is one complete, whole thought, so it is a complete sentence.

Hopefully this was helpful! If you need further help, let me know.

The "I can show you how" would be confusing to add in. If it were clarified as "I can show you how to solve for them." It would be as correct as "Egg yolks contain far more protein and fat than egg whites do." The first sentence is straightforward and gets to the point, while the second sentence doesn't. In other words, the first sentence is a simple sentence  and the second is a complex sentence.

you didn't show the passage, so unfortunately i cant you.