MathsBot is an app that allows you to do calculations, conversions and create charts in a notepad style. Type everything out not only faster than pressing buttons, but it also allows you to see the whole equation and edit mistakes. Everything is saved automatically, so you can continue working from last time. Typing on mobile keyboard can be hard, so this app has a toolbar with basic operators and auto suggestions to make typing faster and easier. MathsBot has eight different themes to choose from, so there is always one that will fit your needs.
