If the number of homogenous products of degree 3 from 4 variables is ab then a+b=

Let four variable are x,y,z,w

We have  $x^a{y}^b{z}^c{w}^d$ such that $a+b+c+d=3$

  No. of non-negative integral solutions  ${ }^{4+3-1}{C}_{4-1}=20$