Math Problem for pharmacy setting

If an adult does is 5mg/kg body weight. What does would we give a 123Ib adult?

Here is what I did:

(5mg/1kg)=(xmg/55.90kg)
x=279.5mg (approximation)

My book says the answer is x= 307.5mg.

What did I do wrong?
I would really appreciate some help!!!
This has been driving me crazy. I have a feeling the book is wrong although.

1 lb = 0.453592kg

(123)(0.453592) = 55.791816kg

If an adult weighs 55.791816kg and the dose is 5mg/kg body weight, then the dose would be:

55.791816*5 = 278.96mg

I agree with you