No, it should reflect the magnitude of the measurement. You wold measure the length of a river in kilometres but its width in metres.
When adding or subtracting measurements, the result should have the same number of decimal places as the measurement with the fewest decimal places. When multiplying or dividing measurements, the result should have the same number of significant figures as the measurement with the fewest significant figures.
In such problems, you should convert everything to the same unit - in this case, to millimeters or to meters. Then it should be easy to compare the two measurements.
they are all the same length
Yes. Because pythagoreans theorem is used to get them by the same method using the same measurements.
When performing addition and subtraction operations with measurements of different significant figures, the result should be rounded to the same number of decimal places as the measurement with the fewest significant figures.
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300 ml is a measure of volume. A decimal is a number - pure and simple - neither mass, nor volume, neither length not time. So there is no decimal that is the same as 300 ml.
To get the capacity (area) of a closet shelf, multiply the length of the shelf by the width of the shelf. Make sure that that both measurements are in the same units. i.e. bithe should be in inches, or feet, or centimeters. If you use different measurements, the resulting value will be wrong.
It could be pear, as the upper measurements are smaller than the lower measurements. If it was hourglass, the measurements should be more or less the same on the top and the bottom.
That's what I call a "square".
Yes. They are merely rounded differently. 12.0cm is rounded to 1 decimal place, and is therefore more accurate.
That's wrong; 18 inch is not a unit of area. In general, since in a square the length and width are the same, you take the square root of the area, to get the length of a side.