King of Maths is a mobile game designed to improve mental arithmetic. It is stylishly presented in the form of a role-playing adventure. Users create a character and must develop their avatar by answering arithmetic questions against the clock.
It was developed by Oddrobo Software (oddrobo.com), and is available on Android and IOS. It is one of the most popular educational games on either platform. A trial version of the game, limited to addition and subtraction of whole numbers, is available for free.
The game is a great way to motivate younger learners to practice their arithmetic skills, but offers little in the way of exposition and would maybe not be as useful for less confident learners.
Resource link: http://oddrobo.com/kingofmath