Possible Duplicate:
Floating point inaccuracy examples

double a = 0.3;
std::cout.precision(20);
std::cout 

result: 0.2999999999999999889

double a, b;
a = 0.3;
b = 0;
for (char i = 1; i 

result: 15.000000000000014211

So.. 'a' is smaller than it should be.
But if we take 'a'  50 times - result will be bigger than it should be. 

Why is this? 
And how to get correct result in this case?