PEMDAS is an advanced scientific calculator with an easy-to-use interface for doing calculations with equations and variables. It is the touch version of PEMDAS Widget, an Apple Design Award winning Dashboard Widget.
The interface of PEMDAS is comprised of several tabs that let you work with equations and variables. The Keypad tab has a number pad, a function pad, and regular keyboard access to let you quickly type and calculate equations. PEMDAS saves all your equations and associated results until you clear them, and the History tab and provides one tap access to duplicating equations or using the results in new equations. PEMDAS auto-assigns variables to all calculations, which you can access in the Variables tab. You can clear the saved equations and results by tapping the clear button in the History or Variables tab, or by pressing and holding the clear button in the Keypad tab.
In addition, PEMDAS lets you switch between degrees and radians, and has support for percentages, trigonometric, hyberbolic, and logarithmic/exponential functions. You can also convert between hexadecimal, octal, binary, and decimal. PEMDAS also has several number formatting options; it lets you format the answer in scientific and engineering notation, has a preference for thousands separators, and lets you limit the amount of significant figures or decimal places displayed.
The website link below contains additional information and also overview videos of PEMDAS.