Question

6.35x10^-3-3.159x10^-2

Answer 1

For addition and subtraction by hand, you need to adjust the mantissa so the exponents are the same. Here, it is convenient to use 10⁻² as the common factor. Then you have

... 0.635×10⁻² - 3.159×10⁻² = -2.524×10⁻²

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A calculator can deal with the numbers as given.