# Thread: Basic probability math (Algebra?)

1. Hey guys.

If you have for example 1000 lottery tickets, 990 with no winnings and 10 lottery tickets with winnings, and you buy 3 lottery tickets.

How do you figure out the probability of winning?

If it was only 1 ticket bought it would be easy, but you have to figure out the modified winning chance per ticket bought?

Math is my bane in life :P Really need some help on what formula to use.

2.

3. Originally Posted by Raziell
Hey guys.

If you have for example 1000 lottery tickets, 990 with no winnings and 10 lottery tickets with winnings, and you buy 3 lottery tickets.

How do you figure out the probability of winning?

If it was only 1 ticket bought it would be easy, but you have to figure out the modified winning chance per ticket bought?

Math is my bane in life :P Really need some help on what formula to use.
Don't think of it as "what formula do I need?" Sit down with a nice cup of tea and try to reason it out.

With probability problems, it is often fruitful to consider the probability of the outcome not happening. Subtract that from 1, and you get the answer to the original problem. That simple shift in point of view quite frequently does the trick for you (I'm not making any guarantees about the specific problem you've presented, but...).

Try things of that general ilk and see if that unblocks you a bit.

4. First, you can calculate the probability of NOT winning. The probability of picking three non-winning tickets is (990/1000)*(989/999)*(988/998) = 0.9703 (not exactly, but I can't type an approximately equals sign). That is the probability of NOT winning, and thus the probability of winning is about 0.0297 or 1 in 34. I assume you are good enough at probability to know where the multiplying numbers came from, but write back if not.

5. Originally Posted by anticorncob28
I can't type an approximately equals sign

Or \approx in TEX tag:

6. Thanks for the tip. I don't understand how the forum really works, in fact the ENTER button doesn't work. I can't type paragraphs.

7. Originally Posted by anticorncob28
Thanks for the tip. I don't understand how the forum really works, in fact the ENTER button doesn't work. I can't type paragraphs.
Click on and highlight the left-most Icon on the posting header; A/A before beginning your post.

jocular

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