Find the number of positive integral solutions of ${\mathrm{x}}_1{\mathrm{x}}_2{\mathrm{x}}_3=30$

30= 2x3x5 =(prime factors) 
We can assign 2, 3, 5 to any one of the 3 variables 
So, each can be assigned in 3 ways 
By product rule, the number of integral solutions = 3³ = 27. 
This is just like to post 3 different letters in 3 boxes.