A wildlife biologist examines frogs for a genetic trait he suspects may be linked to sensitivity to industrial toxins in the environment. Previous research had established that this trait is usually found in one of every five frogs. He collects and examines 20 frogs. If the frequency of the trait has not changed, what is the probability that he finds the trait in (a) none of the 20 frogs? (b) at least three frogs? (c) four or five frogs? (d) no more than six frogs?