Internal energy of an ideal diatomic gas at 300K is 100 J . Out of this 100 J,

Potential energy of an ideal gas is zero because inter molecular forces between them is zero. Degree of freedom of an ideal diatomic gas is 5. In this 2 is rotational and 3 is translational. According to  law of equipartition of energy  internal energy is equally distributed in all degrees of freedom.
Therefore, rotational kinetic energy =  $\frac{2}{3}\times 100=40J$
               Translational kinetic energy =  $\frac{3}{5}\times 100=60J$