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Replace Calculator with PCalc
If all you expect from your calculator is a bit of adding and subtracting, then the built-in Calculator app is fine, but if you want a little more you need PCalc Lite.
PCalc Lite gives you all the power of a scientific calculator on your iPhone or iPad, including unit conversions and constants. If you want more features -- such as multi-line, engineering features, or programmer features -- they can be purchased separately.
Image source: TLA Systems.