Why is this multiplication not working?

[snippsat]

instead of getting .3, I'm getting .3000000000004.

It's the way floating point arithmetic work,Basic Answers Python doc.
There is a Decimal module.

>>> 0.4 - 0.1
0.30000000000000004

>>> from decimal import Decimal
>>> Decimal('0.4') - Decimal('0.1')
Decimal('0.3')