Probabilitiy quenstions

[robinphilip01]

Hi,
The question:
" In a box with 10 blocks, 3 of which are red, what is the probability of picking out a red block on each of your first two tries?Assume that you do not replace the first block after you have picked it."

Answer as -1/15
sum of 3/10 and 2/9.

3/10- chance for first pick
and 2/9 - as chance for second pick.
my doubt is the '2' here is if only the first picked block was red.
other wise, is it remains as 3 itself ?
to become the chance of second pick as 3/9?
or, how can we be sure he/she picked the red itself?

[tommywallach]

Hey Robin,

First off, you meant the PRODUCT of those two fractions, not the SUM.

Now, to your main issue. Remember that when you work out probabilities, you only EVER deal with successes. The first fraction you wrote there was 3/10. Remember, that is the odds of successfully picking a red block. So in answer to your question, "how can we be sure s/he picked the red block," we are sure because we calculated the probability that s/he picked the red block!

So there's a 3/10 chance of success the first time, and if you've succeeded there, you have a 2/9 chance of succeeding again.

Hope that makes sense!

-t