Just a Simple Routine

If you follow my rule with fifteen,
Then you'll surely get seventy-four.
The routine can then also be seen
With what number you wish--say a score.

With a score you get seventy-one,
But with five just an eighty you'll see.
So perhaps you can figure for fun
What the answer with thirty will be.

(Note: A score is twenty.)

(Hunter, J.A.H., (1956). Fun with Figures. New York: Dover Publishers, Inc.)