Double precision number problem. Why 20.5 SOMETIMES is stored as 20.499999000?

[frelek . m]

I am using CW 5.0 without the patch and PalmOS 3.0.

when I compile the following code:
     double var ;
     var=20.5 ;

during the debug, most of the time I see that var=20.4999990000, but
sometimes the right value (20.5) is stored.

What is causing this mismatch and how to prevent it?
Is there any document where double float operations on PalmOS would be
explained?

FYI,
I use 8-byte double setting of the Compiler
I am not using NewFloatMgr


Marcin Frelek