Arabic numerals
The first mentions of the numerals in the West are found in the "Codex Vigilanus" of 976.
Fibonacci
Fibonacci travelled around the Mediterranean coast, meeting with many merchants and learning about their systems of doing arithmetic. He soon realised the many advantages of the Hindu-Arabic system, which unlike the Roman numerals used at the time, allowed easy calculation using a place-value system. In 1202, he completed the "Liber Abaci" ("Book of Abacus" or "The Book of Calculation") which popularized Hindu–Arabic numerals in Europe.