Last year we offered an online Factoring Bootcamp course using the IXL system for our assessments. Their program is excellent. The problems are good, and they gradually increase in difficulty as students progress through the topic. If a student misses a problem, the program backs them up to an easier version of the problem. And, remediation built into the system; each problem is accompanied by three or four suggested problems for those students that feel they are not ready to tackle the given problem. The look and feel of the website is also excellent. You can see the Factoring Bootcamp class itself, and the links to IXL, at www.mathtvcourses.com.
I bought access for up to 25 students in the course, then, as students enrolled, we had them pay $10 for the course. My only problem was that I needed to add them to the roster manually. Then IXL would generate a username for each student and I would have to email them individually because each username was unique. If there was an easier way to get students to pay us for access, and then enroll in the course at the same time, I would recommend IXL to anyone in higher education who is teaching mathematics online. Of course, you can still recommend the website to your students, and even point them to the problems you want them to work on. If they want their scores recorded, so they can see their progress, they would need to pay for an account on IXL.
Overall, a great site for online homework problems. Here is a link to the site: https://www.ixl.com/
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