Here some other substance (e.g. O2) may be involved in the reaction of A and BO3 .
So, in this type of questions, we use POAC (Principle of Atom Conservation).
There are 3 atoms of A in A3O4, i.e. 3 atoms of A react to form one molecule of A3O4.
Similarly, 2 molecules of BO3 react to form 1 molecule of B2O3.
3 A + 2 BO
3 
A
3O
4 + B
2O
3No. of moles of reactants : 1 1
No. of moles of reactants /
stoichiometric coefficient : 1/3 1/2
No. of moles of
substances after reaction : 0 1/3 1/3 1/3
We notice that 1/3
Now, as 3 moles of A react with 2 moles of BO3, 1 mole of A will react with 2/3 mole of BO3.
1 mole of A will produce 1/3 mole of A3O4 and 2/3 mole of BO3 will produce 1/3 mole of B2O3.
