2 commits both passed each other in 1980 one orbits the Earth once every 75 years the other has an orbital period of 21 years when is the next year they will both pass together
The orbits of the two commets should coincide again in the year 2505 A.D.
The lowest common multiple of 21 and 75 is 525 so add 525 to 1980 (to get 2505 as Guest has stated above).
