One - the trailing zeros are not significant.
One. The zeros are placeholders.
400000
Assuming you are dealing with whole numbers only: Half of the numbers round up and half round down. Half of ten thousand is five thousand. Therefore 400000 ± 5000 round to 400000 to the nearest 10000; except that numbers exactly half way round up, so the maximum is one less than this. → smallest to round [up] to 400000 is 400000 - 5000 = 395000 → largest to round [down] to 400000 is 400000 + 5000 - 1 = 404999
100 centimetres = 1 metre so 400000 cm = 400000/100 = 4000 metres. Simple!
231.57 has five significant figures/numbers. All the numbers in 231.57 are significant.
400000 cubic meters = 105600000 gallons US
about 400000
50
60% of 400000 = 60% * 400000 = 0.6 * 400000 = 240,000
The figure 18.03 has a total of four significant numbers
400000
248,548.48mi