Why do simple math operations on floating point return unexpected (inaccurate) results in VB.Net and Python?

Question

x = 4.2 - 0.1  

vb.net gives 4.1000000000000005
python gives 4.1000000000000005

Excel gives 4.1<

Answer 1

Float/double precision.

You must remember that in binary, 4.1 = 4 + 1/10. 1/10 is an infinitely repeating sum in binary, much like 1/9 is an infinite sum in decimal.