An iron ball and cork ball of the same radius are released from the same height in a vacuum both of them reach the ground simultaneously. It is because

Acceleration due to gravity is given by:
$g=\frac{GM}{R^2}$ , 
Where M is the mass of the planet, R is the radius of the planet and G is the gravitational constant. As we can see, acceleration due to gravity is independent of the mass of an object falling and its size or shape. It is the same for all the objects falling on the earth.
Usually, we see that heavier objects fall more quickly than lighter objects. This is due to the resistance of air. Air resistance is more for lighter objects due to which they fall slowly. But in a vacuum, air is absent due to which there is not resistance to the motion of the objects.
Therefore, when an iron ball and a cork ball of the same radius are released from the same height in a vacuum both of them reach the ground simultaneously.