Why pyton don't geav step 0.1 in this program?

Question

>>> i = 3
while i >=0:
print(i)
i = i - 0.1

Answer 1

Due to the way floats (decimals) are represented, they aren't quite precise. Think of it as how we can't represent 1/3 as a decimal (0.3333333333...). I'd recommend either using i = round(i - 0.1, 1) or using i = 30, print(i / 10), i = i - 1

Answer 2

Well, thank you for answer... your recommend i=30 work... but! WHY?!
When program run:
i=30
while i>=0:
print (i/10)
i=i-1

It is positive (0.1, 0.0 at the end). But if just run:
print (1/10)
It is negative! 0.100000000000001.

Or another...
If 3-0.1 (3-1/10) then 2.8999999999999, but if (3-0.1)+0.1 ((3-1/10)+1/10) then 3.0
Why?!

How to make serious calculations in this program or how to program a simple calculator in this case?