Explanation:

Astronomers used data from NASA’s Kepler Space Telescope to estimate how many planets are orbiting red dwarfs in the Milky Way galaxy. They found that about 1 in 5 red dwarfs has an Earth-size planet in its habitable zone. The habitable zone is the range of distances from a star where liquid water could exist on the surface of a planet.

Here is the calculation:

Kepler observed about 200,000 red dwarfs.

About 10% of red dwarfs have an Earth-size planet in their habitable zone.

Therefore, about 1 in 5 red dwarfs has an Earth-size planet in its habitable zone.