In 1961, radio astronomer Frank Drake, working at the Greenbank observatory, proposed a method of estimating the number of civilizations in our Galaxy that could be detectable from Earth. It is written as an equation to determine the possible number of planets that have intelligent life. This equation is basically constructed as a product of two parts.
Number of planets with intelligent life = Number of possible planets x various probabilities
The number of possible planets involves counting a number of important factors such as the rate of star formation and the is the mean lifetime (in years) of a communicative civilization. The various fractions are many, but all practically selected. They include the fraction of stars with planets, planets with suitable environment for life, inhabited planets where intelligent life-form evolves, planets with intelligent life that has developed radio communications, and others. This equation can be applied to our galaxy, the Milky Way, or to other galaxies, or even to the entire universe.
Note the use of percentages and quantities in the same equation.