JEE Advanced 2008 Paper 2Question 1
An experiment has 10 equally likely outcomes. Let $A$ and $B$ be two non-empty events of the experiment. If $A$ consists of 4 outcomes, the number of outcomes that $B$ must have so that $A$ and $B$ are independent, is
Probability
Solution
Independence needs $n(A\cap B)=\frac{4\cdot n(B)}{10}$ to be an integer, so $5\mid n(B)$, giving $n(B)=5$ or $10$.