If Aryabhata invented zero in Kaliyuga, how could Hindus of earlier yugas describe Ravana as having 10 heads and Kauravas were 100
I feel the your doubt is about representation
If you look at the history of representation, it starts with the story of people representing numbers with straight lines, and crossing them after sometime. Roman Numerals for example did not originally have 0. Multiplication and division might have possibly be done in form of addition and subtraction and if you look closely,you will not feel the importance of zero in them. Its implicit, why would anyone add nothing to a sum, or subtract nothing from the sum, the resultant will remain same, won't it? So simple calculation is not a problem.
Aryabhatt defined the value of 0/0 as 0 which is not in the case of modern definition. So it is possible that there was no such thing as zero and the calculation might have been totally dependent over some other notation as blanks dots etc.When you define something, does it not inherently implies 2 scenarios 1>That the one who defines is either correcting the value of a previously held concept 2>He might possibly be defining his own standard to facilitate correct calculation. **
Hence it is difficult to jump to conclusions and say that zero was there during the time when ramayana or mahabharata was written.
Here is an excerpt from wikipedia: The concept of zero as a digit in the decimal place value notation was developed in India, presumably as early as during the Gupta period (c. 5th century), with the oldest unambiguous evidence dating to the 7th century. The Indian scholar Pingala (c. 200 BC) used binary numbers in the form of short and long syllables (the latter equal in length to two short syllables), a notation similar to Morse code. Pingala used the Sanskrit word śūnya explicitly to refer to zero. The earliest text to use a decimal place-value system, including a zero, the Lokavibhāga, a Jain text surviving in a medieval Sanskrit translation of the Prakrit original, which is internally dated to AD 458 (Saka era 380). In this text, śūnya ("void, empty") is also used to refer to zero. The origin of the modern decimal-based place value notation can be traced to the Aryabhatiya (c. 500), which states sthānāt sthānaṁ daśaguṇaṁ syāt "from place to place each is ten times the preceding", The rules governing the use of zero appeared for the first time in the Brahmasputha Siddhanta (7th century). This work considers not only zero, but negative numbers, and the algebraic rules for the elementary operations of arithmetic with such numbers. In some instances, his rules differ from the modern standard, specifically the definition of the value of zero divided by zero as zero.
So your doubt about calculation of 10 and 100 can be easily attributed to the above fact of simple addition and calculation. Now what remains is the representation part. So representation could be in various bases and various forms in the same base.In Ancient india, prakrit, sanskrit etc were used and they do have a good representation of numbers. If you want an example: Romans use the letter "C" to denote 100 and 'X' to denote 10 in decimal notation. In hexadecimal 100 is written as 64. So it can easily be seen that in the olden days, there might be a whole different representation for numbers with zeros. You can go to the following link to know more if you want. https://en.wikipedia.org/wiki/0_(number) I hope i answer you question. If any doubts, feel free to question me.
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