Can a variable start with a number?If not then why?

Answer 1

A variable name cannot start with a number as the formal grammar by which the compiler parses source code does not allow it. See http://www.nongnu.org/hcb/#identifier , if you can read Backus-Naur-Form (abbreviation: BNF; BNF is a way formal grammar is written down).

Answer 2

No its not possible because it's a constant number.
Generally we take english alphabets for variable.