A container contains hundred eggs. They can be either fresh or rotten. What is sure is the fact that there is at least one fresh egg in that container.
If you are asked to pick two eggs randomly from the container, at least one of them will be rotten.
Can you calculate how many eggs in that container are fresh ?
Only one egg in that container is fresh.
The question tells us that at least one egg in that container is fresh. Then the question says when two eggs are picked at random, at least one egg will be rotten. This concludes that all the other 99 eggs are rotten or there would have been possibility of picking two fresh eggs as well.