Please check out a demo of this project: http://subtract.online
A few features to note:
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The site is intended for child users and complies with the Children's Online Privacy Protection Act. We do not collect personal information. Keeping the registration process as simple as possible, we do not require an email addresses to sign up. Restrictions on usernames and passwords are kept to a minimum: usernames must be unique (validated with ajax) and passwords must be at least 6 characters.
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By default, the program selects 10 problems at a time to work on. This can be changed using the query parameter "problems" once the user has logged in. For example, if my username were "Name" and I wanted to practice only 3 problems, I could go to subtract.online/Name?problems=3
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Additional problems can be selected for practice by clicking on them in the list.
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A problem is "mastered" when it has been answered two times in less than 5 seconds each time.
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The practice period runs until all selected problems have been mastered, or after a set time limit. The default time limit is 3 minutes, but the time limit can be changed using the query parameter "seconds" after the user has logged in. For example, subtract.online/Name?seconds=20
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After a problem has been displayed for 5 seconds, a set of dots appears that can be used to help solve the problem. Each dot changes color when clicked.
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The app keeps a record in the database of how many seconds a user has practiced each day, and how many problems have been answered that day. For now this information is only used in a greeting message, but in the future it could be displayed in a chart or graph.
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For insight into how this app was designed, see this blog post.