In an RC circuit, what fraction of the final energy is stored in an initially uncharged capacitor after it has been charging for 3.0 time constants

The  fraction fraction of the final energy is stored in an initially uncharged capacitor after it has been charging for 3.0 time constants is  

Explanation:

From the question we are told that

     The time  constant  

The potential across the capacitor can be mathematically represented as

Where is the voltage of the capacitor when it is fully charged

    So   at  

   Generally energy stored in a capacitor is mathematically represented as

In this equation the energy stored is directly proportional to the the square of the potential across the capacitor

Now  since capacitance is  constant  at  

        The  energy stored can be evaluated at as

Hence the fraction of the energy stored in an initially uncharged capacitor is