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Have two variables
- long counter
- long sum
use them wisely while iterating through the LL.

at the end print avg=sum/counter

Your question is unclear. Please post your code.

For starters,
a) use random inside the while loop
b) have a counter inside while loop increase till 100 and break

Instead of using a for loop, try to populate a ch[3] with random symbols appearing from a list of symbols in each of the array position.