Answer:
Length, l = 33.4 m
Explanation:
Given that,
Electrical field, 
Let the electrical potential is, 
We need to find the length of a thundercloud lightning bolt. The relation between electric field and the electric potential is given by :

So, the length of a thundercloud lightning bolt is 33.4 meters. Hence, this is the required solution.
20.4 years is 20.4/10.2 = 2 half-life cycles, which means a quarter of the starting mass or 15.2 g will remain after this time.