An urn contains 6 white and 4 black balls. A fair die is rolled and that number of balls are chosen from the urn. The probability that the balls selected are white is:

1/5

Let E_i : Die shows no. i ; W : All balls selected are white.

$P(W)=\sum\limits_{i=1}^{6}P(E_i)P(W/E_i)=(\frac{1}{6})[\frac{6}{10}+\frac{{^6C}_2}{{^{10}C}_2}+\frac{{^6C}_3}{{^{10}C}_3}+\frac{{^6C}_4}{{^{10}C}_4}+\frac{{^6C}_5}{{^{10}C}_5}+\frac{{^6C}_6}{{^{10}C}_6}]=\frac{1}{5}$