Question

The atomic mass of He is 4u. How many 'He' atoms are present in 52 u?

Answer 1

4 amu of He = 1 atom of He. thus 1 amu of He = (1 / 4) atom of He. So 52 amu of He = (52 / 4) atom of He = 13 atoms of He....

Answer 2

4g of He has 1 mole 
1g of He has 1/4 mole 
52g of He has 1/4x52 = 13 moles 

1 mole=6.022x10^23atoms 

OR 

Atomic mass of Helium is 4u = 2protons + 2 Neutrons = 1 atom  
No. of atoms in 52u = 52/4 = 13 atoms