If the events $A$ and $B$ are mutually exclusive,then $P\left( \frac{A}{B} \right) = $

From a well-shuffled pack of $52$ cards,one card is drawn at random. The probability that the card drawn is an ace is

$A$ can solve $90 \%$ of the problems given in a book and $B$ can solve $70 \%$. What is the probability that at least $1$ of them will solve a problem selected at random from the book?

The letters of the word $SOCIETY$ are placed in a row. What is the probability that the three vowels come together?

An urn contains $25$ balls numbered $1$ to $25$. Suppose an odd number is considered a 'success'. If $2$ balls are drawn from the urn with replacement,find the probability of getting at most $2$ successes.

There are $4$ envelopes with addresses and $4$ corresponding letters. The probability that no letter goes into its correct envelope is: