Digital Sum Formula

Please advise a formula for -

I have two columns to record players scores, A,B,

Column B is to show 21 when 0 to 21 is entered in column A, but column B must show nothing until an number is entered in column A .

Object - if a player scores e.g 6, entering 6 in column A makes column B show 21.

Columns A & B can then be totaled to give the team's total score without manually entering a 21 score.

Hope this makes sense to you?

Regards,

daroston