ASSISTments does not provide answer keys. However answers are available to the teacher in the Overview bar in the report.
When accessing these problem details, the teacher can see the problem assigned, the correct answer(s) and a breakdown of student performance.
If the teachers you work with use one of the open source curricula we offer in ASSISTments (EngageNY, Open Up Resources, Illustrative Math), just note that all curriculum materials and answer keys are available on their website for free.