Who invented the decimal notation in arithmetic?

Answer 1

The Chinese or Romans?

Certainly not the Romans.

The concept of decimal fractions was known to the Chinese in the first century BCE (Joseph Needham, Science and Civilisation in China) but in Europe the modern decimal notation originates from the Dutch mathematician Simon Stevin (1548/49 - 1620) although the decimal point comes later and was in use by the time of John Napier.

Answer 2

The romans