Current transformer 400A primary: Calculate load current if ammeter reads 4.1A in CT circuit

Prep to challange am unable to come up with answer or outline of solution to this question.

 

"A current transformer has a primary rating of 400 Amps"

Calculate the load current if the ammeter in the C.T. circuit reads 4.1 Amps

I think there is some important rate or number that i need i just cant think of what it is.