Number of nitrogen atoms present in melamine is x. The number of –imine and –amine groups is y and z. The sum of x, y and z _____. 

Number of nitrogen atoms present in melamine is 6( x) . The number of imine   groups are 3 (y) and –amine groups  are 3 (z) 

The sum of x+ y+ z = 6+3 + 3 = 12