Writing oxidation numbers of all the atoms,
0Zn++1H+5N−2O3→+2Zn(+5N−2O3)2+−3N+1H4+5N−2N−2O3++1H2−2O
The oxidation numbers of Zn and N have changed.
0Zn→+2Zn(NO3)2 .......(i)
H+5NO3→−3NH4NO3 ..........(ii)
Increase in Ox. no. of Zn=2 units per Zn atom
Decrease in Ox. no. of N=8 units per HNO3 molecule
Eq.(i) should be multiplied by 4 and added to eq. (ii),
4Zn+HNO3→4Zn(NO3)2+NH4NO3.
To balance nitrogen, 9 molecules of HNO3 should be added on LHS.
To balance hydrogen and oxygen, 3H2O molecules should be added on RHS. Hence, the balanced equation is
4Zn+10HNO3→4Zn(NO3)2+NH4NO3+3H2O.