價格：免費
檔案大小：15.6 MB
版本需求：需要 iOS 9.1 或以上版本。與 iPhone、iPad 及 iPod touch 相容。
支援語言：英語
A simple and intuitive iOS app that allows users to practice their subtraction skills by providing them with opportunities to count backwards from numbers using challenging intervals.
支援平台：iPhone, iPad