Math42 is described as 'MATH 42 helps over 2.200.000 middle-school, high-school and college students, to solve their math problems and get better grades at the fraction of the cost of a private tutor' and is an app in the Education & Reference category. There are more than 25 alternatives to Math42 for a variety of platforms, including Android, Windows, Online / Web-based, iPad and Mac. The best alternative is SpeedCrunch, which is both free and Open Source. Other great apps like Math42 are Wolfram Alpha (Freemium), Qalculate! (Free, Open Source), Windows Calculator (Free, Open Source) and fxSolver (Free).
SpeedCrunch is a free, fast, high precision open source algebraic calculator with an intuitive interface and a extensive list of features, including: Unlimited Variables, Syntax Highlighting, Functions, with 'Automagic' Completion, 50 Decimal...
Wolfram Alpha is a "Computational Knowledge Engine" - different from a search engine, it answers factual queries directly by computing the answer from structured data, rather than providing a list of documents or web pages that might contain the answer.
Qalculate! is a multi-purpose cross-platform desktop calculator. It is simple to use but provides power and versatility normally reserved for complicated math packages, as well as useful tools for everyday needs (such as currency conversion and percent calculation).
A simple yet powerful calculator that includes standard, scientific, and programmer modes, as well as a unit converter. It's the perfect tool to add up a bill, convert measurements in a recipe or other project, or complete complex math, algebra, or geometry problems.
Soulver is a smart notepad with a built-in calculator. It gives instant answers to any calculations it finds in your text. It's a better way to work stuff out than with a traditional calculator, and a more lightweight tool for quick calculations than a spreadsheet.