Simply open any of your txt files or use an empty new one. Enter 2 plus 3 and press Enter.
The result = 5 will be added automatically:
2 + 3 = 5

Use any of +, -, /, * operations:
-2 + 3.5 = 1.500000
1 + 2 + 3 - 4 = 2
2 * 3 = 6
8 / 4 = 2.000000
1 * 2 * 3 * 4 = 24
8 / 2 / 2 / 2 = 1.000000

Change any of entered numbers and press F5 (or button Refresh). The whole content will be recalculated according your changes.
You can save the document as a txt file or print out the document as usual. Then you can open any of saved files, make any changes and recalculate the content. Please add the extention (for ex. .txt) to the name of file while saving it.
You can use any comments or texts inside file:

Expensies for August:
income minus for wife minus for home minus for children minus for car is:
1000 - 100 - 200 - 300 - 150 = 250

You can use math functions:
sin(30) = 0.500000
cos(60) = 0.500000
tan(45) = 1.000000
asin(0.5) = 30.000000
acos(0.5) = 60.000000
atan(1.0) = 45.000000
sqrt(16) = 4.000000
exp(1) = 2.718282
log10(10) = 1.000000
log(100) = 4.605170
fabs(-1.) = -1.000000
sinh(30) = 0.547853
cosh(60) = 1.600287
tanh(45) = 0.655794

To switch arguments to radians (for sin, cos etc), press the button Radians. The whole content will be recalculated.
You can open a lot of documents (files) together. Use menu File / New to open a new document. Use menu Window / Tile to arrange all opened documents.

Tipps

You can mix [+ and -] or [* and /] operations:
-1 + 2 - 3 + 1 = 0
4 * 2 * 1 / 2 * 2 * 2 = 1.000000

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