If we have a total of 81 astronauts, and 43 of them are in outer space, the number of astronauts on Earth is 81 - 43 = 38 astronauts
The ratio of the number of astronauts on Earth to the total number of astronauts can be calculated just dividing the two values: in the numerator, the number of astronauts on Earth, and in the denominator, the total number of astronauts. So: