Calorie Burning Question

I used to know this fact off the top of my head but since it really means nothing I’ve forgotten the numbers.
Can anyone help me out.

you must burn X number of calories to lose or burn off X amount of fat.???

A client at my gym asked me this and I’m shocked that I can’t remember.
Does anyone know the answer?

Thanks in advance.

There is no number like that. It is more complicated. But, there are 3500 calories in a pound of fat.