To round a number to a particular digit, look at the digit immediately to the right of your target. If that digit is 4 or lower, zero it and everything to the right of it out. If that digit is 5 or higher, increase the target digit by one and zero everything to the right of it out. If your target digit is a 9, increasing it will turn it to zero and increase the digit to the left of it by one.
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100 is already rounded to one significant figure.
It then is 40000 to one significant figure
10000000
9000
It is 10