Why did the romans choose x to represent 10?

Answer 1

The origin is not known for certain, but one school of thought is that the numbering system evolved from an Etruscan tally stick counting system. The symbol I therefore represents a single notch cut into wood, not the letter I it later became.

To make it easier to count the tally just by running your finger over the notches, every 5th notch was double cut, creating ⋀ , while every 10th notch was cross cut, creating an X.

The Roman alphabet shares I and X, but not ⋀, so it was probably the Romans who introduced the V.

The system continues with L on every 10th ⋀, which probably began as something like ⋔, evolving into ⊥ to finally evolve into an L.

Every 10th X probably began as Ж (an X with a vertical notch superimposed) which probably became a variation on Ɔ|C which was then abbreviated to just C.

Every 10th L was probably something like Þ or Ð, which eventually became D.

Every 10th C was probably something like ⊗ or ⋈ that eventually became an M.