$A$ coin is tossed twice. What is the probability that at least one tail occurs?

Six faces of an unbiased die are numbered with $2, 3, 5, 7, 11$ and $13$. If two such dice are thrown,then the probability that the sum on the uppermost faces of the dice is an odd number is

$A$ card is drawn randomly from a pack of $52$ playing cards. The probability that the card drawn is neither an ace nor a king is:

The probability that a person $A$ completes a work in a given time is $\frac{2}{3}$ and the probability that another person $B$ completes the same work in the same time is $\frac{3}{4}$. If both $A$ and $B$ start doing this work at the same time,then the probability that the work is completed in the given time is

If $A$ and $B$ are mutually exclusive events with $P(A)=\frac{1}{4}$ and $P(B)=\frac{3}{7}$,then what is the value of $P(A / A \cup B)$?

For independent events $A_1, A_2, \dots, A_n$,$P(A_i) = \frac{1}{i + 1}$ for $i = 1, 2, \dots, n$. Then the probability that none of the events will occur is: