List of top Mathematics Questions on Conditional Probability asked in WBJEE

n objects are distributed at random among n persons. The number of ways in which this can be done so that at least one of them will not get any object is

There are two coins, one unbiased with probability $\frac{1}{2}$ of getting heads and the other one is biased with probability $\frac{3}{4}$ of getting heads. A coin is selected at random and tossed. It shows heads up. Then the probability that the unbiased coin was selected is