Number PJ

What is the inverse of any integer between 1 and 20 ?

The answer is

One two buckle my shoe
Three, four, knock at the door
Five, six, pick up sticks
Seven, eight, lay them straight
Nine, ten, a big fat hen
Eleven, twelve, dig and delve
Thirteen, fourteen, maids a-courting
Fifteen, sixteen, maids in the kitchen
Seventeen, eighteen, maids in waiting
Nineteen, twenty, my plate's empty

Don't get how? These are numbers in verse. (inverse)