Here's a couple of easy exercises:

1. How many primes does one expect to find in the class k*10^n-1, where 2<=k<=9 and 1000000<=n<1250000.

Hints for the small adjustments of probabilites:a) k is in {2,3, 5,6, 8,9}

b) for k=8, remove n :: 3|n; for k=9, remove n :: 2|n

2. 10% of the range was checked and 1 prime was found. How many more primes does one expect to find?