Quote (frail @ Nov 7 2011 10:37pm)
Having 10 tickets that each only have 6 numbers yields a lower probability of winning than having 1 ticket with 10 numbers. Consider having 45 tickets with 6 numbers each, and 1 ticket that has all 45 numbers. Obviously, the 1 ticket with 45 numbers has a 100% chance of winning, while the 45 regular tickets have a far lower winning rate.
The probability of guessing the 6 correct numbers with only 6 guesses is
Code
6 5 4 3 2 1 6! 6!(45-6)!
__ * __ * __ * __ * __ * __ = ___________ = _________
45 44 43 42 41 40 45!/(45-6)! 45!
This much you have calculated correctly. But this next step is where your mistake was.
The probability of guessing the 6 correct numbers with 10 guesses is equal to the above probability multiplied by the number of ways that the 6 winning numbers can be arranged within 10 guesses.
Code
6!(45-6)! 10! 6!(45-6)!10! (45-6)!10!
_________ * _________ = ____________ = __________
45! 6!(10-6)! 45!6!(10-6)! 45!(10-6)!
Seems to me your probability is...thousands of tickets, and your final answer seems to reflect that. I can't imagine the odds of winning the lottery with 10 tickets being around 1/40,000. That's way too low.
I don't see how buying 10 tickets increases your chances from one in roughly 8,000,000 to one in roughly 40,000
Similarly, you wouldn't buy (45!-(45-6)!)! tickets for a guarantee of winning. You'd buy (45!-(45-6)!) tickets.
Out of curiosity, I plugged 20 into your formula and the probability now is roughly one out of 200. 30 tickets yields a probability of seven out of 100. 40 tickets is basically a coin flip.
Considering the payout, I don't think a 50/50 chance of winning with 40 tickets is economically plausible for the lottery.