Minimum number of times a fair coin must be tossed so that the probability of getting at least one head is more than 99% is :

Assume that each born child is equally likely to be a boy or a girl. If two families have two children each, then the conditional probability that all children are girls given that at least two are girls is :

Four persons can hit a target correctly with probabilities $${1 \over 2}$$, $${1 \over 3}$$, $${1 \over 4}$$ and $${1 \over 8}$$ respectively. if all hit at the target independently, then the probability that the target would be hit, is :

The minimum number of times one has to toss a fair coin so that the probability of observing at least one head is at least 90% is :