Walter,

Well, that's not exactly how it is calculated, but it is not a bad estimate.

The easiest way to calculate the probability of winning at least one prize in a multi-prize raffle
is to calculate the probability of winning no prizes at all, and then subtract that value from 1:

Now, for the first draw, since you have 20 tickets, the probability of NOT winning the prize for
that draw is 1980/2000, or .99. But for the next, there are 1979/1999 ways to not win, or a
slightly less chance. If you extend that for 15 times, and multiply the results together, you get
0.8596, which means you have a 0.1404 chance of winning, or about 1 in 7 - very close to your
estimate.

But you can see that your estimate is better with fewer tickets. If you had 134 tickets, your calc
would have given a number greater than 100%, when in fact, you would have about a 75% chance of
winning at least once.

HTH,
Bernie
MS Excel MVP


"Walter Mayes" wrote in message
...
This is off topic for this group but was hoping someone could give me an answer.

Total tickets sold: 2000
Chances to win: 15
Tickets held: 20

Would my chances of winning a prize simply be 2000/15/20= 1 in 6.667 ?

The above seems logical but a little too simple. :-)

Walter Mayes