Because
1/3
is computed using the integer division. try
#include <stdio.h>
int main()
{
int a;
float b;
b = 1.0f / 3 + 1.0f / 3 + 1.0f / 3;
a = (int) b;
printf("%d, %f\n",a, b);
getchar();
return 0;
}
And, please, stop using turbo C (it was good about 30 years ago). Use a modern compiler.