For this problem, Benford's law should be known (Benford's law - Wikipedia, the free encyclopedia).

The problem:

Assume that Benford's law is true in the following situations.

a) In a list you find 125 amounts. Caculate the probability that less than 20% of those amounts begin with a 1.

b) Every day, a warehouseman fills out a form with the number of transactions he performed that day. At the end of a quarter he has submitted 80 forms. Calculate the probability that on those forms, at least 10% of the transactions begin with a 9.

I don't really know how start tackling these kind of problems. Does anybody have a good resource to practice the needed skills for these problems? And can somebody explain how to do these problems? I have practiced permutations, combinations, and basic probabilities (like number of successes divided by total number)

Thanks!