Precision Loss due to multiplication issue in siebel escripting

In BC PrewriteRecord event, there is a code like

var n = a*b;

Consider,

a = 0.1, b = 3,  then n=a*b is getting calculated as 0.30000000000000004  instead of 0.3. Please explain how it is getting calculating like this.

Even in siebel bookshelf itself, there is an example

var n = 34.855;
n = n* 100;
var r = Math.round(n)

The value of n is 3485.499999999999995 instead of 3485.5.