A free app for Android, by MathGen.

The K-math application is a free, educational, and fun app that helps children and adults learn to count, calculate, and do simple arithmetic. It contains all the mathematical operations you need to master elementary school math, including multiplication, division, addition, and subtraction. You can even perform all calculations by using either the correct numbers or your own, so you can learn how to do long division, and how to calculate the square and cube of numbers. This application can be used by children, teenagers, and adults. It is ideal for those who want to learn to count and do simple arithmetic.