Mia purchased a corporate bond at its face amount of $10,000 on July 1, 2023. The bond pays 5% interest on each June 30. On September 30, 2023, Mia sold the bond for $10,600. Mia must recognize $125 of interest income in 2023 (5% $10,000 3/12). She must also recognize a $475 capital gain from the sale of the bond:

My question is where did 3/12 come from? What does it mean?