In a precision bombing attack, there is a `50%` chance that any one bomb will strick the target. Two direct hits are required to destroy the target completely. The number of bombs which should be dropped to give a `99%` chance or better of completely destroying the target can be

A rifleman is firing at a distant target ansd hence, has only 10% chances of hitting it. Find the number of rounds, he must fire in order to have more than 50% chances of hitting it at least once.

Assume that the chances of a patient having a heart attack is 40%. It is also assumed that a meditation and yoga course reduce the risk of heart attack by 30% and prescription of certain drug reduces its chance by 25%. At a time a patient can choose any one of the two options with equal probabilities. It is given that after going through one of the two options the patient selected at random suffers a heart attack. Find the probability that the patient followed a course of meditation and yoga?