Then known number in an algebraic expression is

Unsure of what you're asking.  

If you have an algebraic expression in front of you and see a number there, you could label it as "known" or "a known quantity."  Another word that comes to mind is "constant."

No he does not because he can only cover 12 square feet from 15 square feet which he will still need 3 more. this is because if 3×5=15 square feet and 15-12=3 which thats the answer