calculate the percentage by mass of ozygen in trioxnitrate5 acid H-1, N-14, O-16

Answer 1

HNO3.
mass HNO3 = 1+14+(3*16) = about 63.
mass O = 3*6 = 48
%O = (mass O/mass HNO3)*100 = ?

Answer 2

Replace mass O = 3*6= 18
with mass O = 3*16 = 48