8.4 x 106
Basically, since this is all on the left side of the decimal point, you take the first 2 digits, round them (need to look at the 3rd digit '9' for rounding), then you set the 10n exponent , where n is the number of decimal places it takes to get from the shortened number to the original number (rounded)
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272.0 rounds to 270
It is: 3.4 because .01 is less than .05
Any measurement may have two significant digits.
When using significant digits, the product has only the number of significant digits as the lowest number in the factors. "20" has two significant digits and "310" has three. Therefore, the product has to have two significant digits. 310 × 20 = 6200 6200 already has two significant digits.
For a quick estimate, you would usually round to one, sometimes to two, significant digits. One significant digit means discarding all digits after the first, i.e., converting them to zero (and rounding the remaining digit up or down as appropriate).