1. The moon has been in existence for the 4.6 billion years.
2. The moon has been in existence for 4.6 billion years.
Are both correct? If not, please explain to me.
#1 is not natural English. 5 billion years.
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#1 is not natural English. It would only make a little bit of sense in some unusual context where you had already mentioned 4.5 billion years.