Answer:
20 Students attended
Explanation:
This question is weird... you need more detail to answer this as even though 20 students danced, some other students could have just been sitting down. However, because we are not given that information I'm going to assume that the people who attended the dance actually danced.
Thus 20 students danced.
Also, if there is an answer choice saying "not enough details" I would pick that instead.