Because 3× 1/3 is an infinite cyclic decimal equivalent to 3× 0.3, because the infinite cyclic decimal ring is 0.3= 1/3, 3× 1/3= 1,

Similarly, 9× 1/9 is the same.

Son, you study math, not math. It is good to be diligent in thinking, but don't complicate the problem. These axioms have been formulated by other mathematicians and belong to the outside world. There will be others (especially chemistry) in the future. If you insist on a dead end and solve the accidents that scientists can't explain in those years, you won't reach the excellent learning level.