Put a problem of the day on your site
One iframe, no key, no account, no script of ours on your page beyond the frame. It shows one problem from the skill you choose, changes daily, and carries a link back.
The snippet
<iframe src="https://sumcomb.com/embed/problem?skill=long-division-3-digit-by-1-digit-with-remainders&ref=embed" width="100%" height="230" style="border:1px solid #d6d2df;border-radius:3px" loading="lazy" title="Problem of the day"></iframe>
<p><a href="https://sumcomb.com/?ref=embed">Problem of the day from SumComb</a></p>Change skill= to any of the 339 slugs in the catalogue — the slug is the middle part of a practice address. Keep the link under the frame: it is the only thing we ask for.