Please help me to solve this Problem: We generate random 4-character receipt codes where each character may be any of this 28 letter and digits: 23456789BCDFGHJKMNPQRSTVWXYZ Therefore there are 28 x 28 x 28 x 28 = 614656 possible receipt codes. If I generate 50,000 receipt codes, how many repeated codes should I expect (how often should a code appear more than once)?? How do I calculate myself for numbers other than 50,000?