Confused again ok I have to find the number of atoms in carbon with a mass of 13.5. So this is what I didi.

13.5 / 12.01* 6.022 *10^23= 6.77x10^23 but the correct answer is 6.74x10^23 what have I done wrong

If that is 13.5 GRAMS, you didn't do anything wrong. 6.77E23 is correct.