To calculate the percentage composition we need to know the molar masses first.
Ammonium oxide (NH4)2O = 70.09 g/mol
H = 1.00784 g/mol
N = 14.0067 g/mol
O = 15.999 g/mol
To find the % of H (Remember we have 8 hydrogen in the formula)
H% = [(1.00784 x 8)/70.09] x 100
H% = 11.5%