[Tutor] round() yields in-expected results/commercial rounding

[Harm_Kirchhoff at mail.digital.co.jp]

I have the following problem:

>>> float(8.7)
8.6999999999999993
>>> float(870)/100
8.6999999999999993
>>> 8.7 * 100
869.99999999999989
>>> round(8.7,2)
8.6999999999999993
>>> math.modf(8.7)
(0.69999999999999929, 8.0)
>>> int(float(8.7*100))
869

especially anoying is:
>>> n = float(8.7*100) ; print n
870.0
>>> n
869.99999999999989
>>> 

How can I ensure that 8.7 stays 8.7, so that
8.7 * 100 = 870 and not 869.9999...

The way I came up with is:
>>> f = float(8.7*100)   ;  i = int(f)   ; i,f
(869, 869.99999999999989)
>>> if f>=(i+.5): i += 1 ;  i

870

I am writing a commercial sales database. To save memory I use integers 
internally, rather than floats.
This means that an amount of 8.70 is converted to 870 internally.
if it is converted to 869 I loose 1/10.