Problem calculating relative molecular mass [closed]

Question

I am starting be re-learn Chemistry after several years.

The Book I am using has a test question to calculate the relative atomic mass of:

$ (CH_3CO)_2O $

My take

Carbon C, $ A_r = 56 $
Hydrogen H, $ A_r = 1 $
Oxygen O, $ A_r = 16 $

$ M_r = ( 56 + 1 \times 3 + 56 + 16 ) \times 2 + 16 = 131 \times 2 + 16 = 278 $

This is nothing like the answer given in the answer key. Is there an error in my text book?

If not where am I going wrong?

---

Edit: Problem was a misprint in my text book. See correct answer below

Answer 1

Solved. The problem was a typing error in the text book $ A_r $ for carbon should of course be 12 not 56. This is given correctly in the periodic table shown on the back cover but was misprinted in the body of chapter 1 of my book.

Thanks to Zenix for pointing this out in comments.

$ M_r $ of $ (CH_3CO)_2O $

Carbon C, $ A_r = 12 $
Hydrogen H, $ A_r = 1 $
Oxygen O, $ A_r = 16 $

$ M_r = ( 12 + 1 \times 3 + 12 + 16 ) \times 2 + 16 = 43 \times 2 + 16 = 102$

This matches the answer given in the answer key.

I have not studied chemistry in over 30 years which explains why 56 as a relative atomic mass for Carbon did not scream out as being wrong.