Notice that in the equation given, there is 1 mol of Mg3N2
and 6 mol of H2O. Consequently, the question is requesting, if we took 2.5 mol
of Mg3N2, how many mol of H2O would be essential to complete a similar reaction
-- in quintessence, multiplying everything by 2.5:
the solution would be: 6 * 2.5 = 15 mol
