If the oort cloud contains 1012 comets, and ten new comets are discovered coming close to the sun each year, what percentage of the comets have been “used up” since the beginning of the solar system?

4.57 %

Explanation:

The age of the solar system in years is about

Each year 10 new coments enter the solar system, so the total number of comets "used up" since the beginning of the solar system is

The number of comets contained in the Oort cloud is

Therefore, the percentage of comets that have been "used up" is

answer: electrons