Final answer:
The logical fallacy in the sentence about Sally being a terrible class president due to her saxophone skills is an irrelevant conclusion fallacy. This is an example of an informal fallacy where the premises do not logically support the conclusion.
Step-by-step explanation:
The logical fallacy in the sentence 'Sally would make a terrible class president. She may be good at debates, but she is a terrible saxophone player!' is an irrelevant conclusion fallacy, sometimes known as ignoratio elenchi. This is because the skill of playing the saxophone has no relevance to the qualities needed to serve as a class president. The argument diverts attention away from Sally's relevant skills, like being good at debates, which may contribute well to her potential role as a class president.
In logical terms, an informal fallacy occurs when the conclusion doesn’t follow from the premises. In contrast to formal fallacies, which stem from an error in the logical structure of an argument, informal fallacies often originate from a mistake in reasoning due to incorrect connections between premises and the conclusion. Examples of informal fallacies include ad hominem, bandwagon, and bait and switch. In Sally's case, her being a terrible saxophone player is not logically connected to her capability of being a class president, hence it's an irrelevant point in the argument.