# Probability: best chance of picking a desired marble out of 10

following are the two questions I've made myself, but I need help in solving them.

1) Suppose there are 10 marbles in a box. One out of them is the desired marble, or you can say one is black others are white,etc etc. Case 1 : You pick 3 marbles altogether out of the box. What is the probability that you'll choose the black marble ? Case 2: You pick 3 marbles out, but one at a time. Now what is the probability of getting the black marble in those 3 attempts ?

2) Now suppose there are 10 marbles arranged linearly, each with position say 1,2....10. again, one is black & rest are white.You are to pick 3 marbles in order to have best chances of getting a black marble. Mathematically, is it better to pick 3 successive marbles,ie position 3,4,5 or would it be better to pick up randomly, like position 3,6,9 or it makes no difference whether you do it either way ??

Note: Although both questions can be put into one single question, it'd be better to solve it, & for me to understand it, if put up as two seperate questions.

-
my answer to Q1. case 1: 0.3 & case 2: 0.27 – Sumedh Oct 5 '13 at 12:03