Sara tells Kate that she will give her $50 to clean her garage. When Kate is halfway done, Sara decides to revoke her offer. Is this a valid revocation?

No, because this is now a bilateral contract

Explanation:

If an offeree/ a promisee has started performing the said work, it becomes a bilateral contract between the offeror/ promisor and the offeree. It cannot be revoked until the completion of the performance.

A contract is an agreement between two or more people that is legally binding to both parties involved. It is usually oral or written and it is mutually bindings obligation. A contract contains the enforcer whicha can be a terms and conditions, an offer, a performance and others.

A bilateral contract is an agreement between two parties whereby both parties agrees to fulfill its own aspect of the bargain. Example is any sales agreement, a work agreement such as i eill pay you $70 dollars if you can supply me 2 crates of egg. A breach of contract is when either of the parties involved in the contract did not fulfilled his or her part of the contract e.g Offer by the promisor not fullfiled or Acceptance/ work by the promisee is not fulfilled.

what is the tax base for the sales tax?

it should be (usually ) 6%