r + j + c + L = 30    (L -cLassical; the remaining notations are clear)

2r = 3c

j = L

================>

r + j +  + j = 30,

3r + 3j + 2r + 3j = 90

5r + 6j = 90

j = 


You need to choose the "free" value of r in a way 

    a)  r  is integer >= 0,   and

    b)    is  integer >= 0,   and

    c)   is integer >= 0.


Table

    r    j =(90-5r)/6    c =    L = j
---------------------------------------------------
    0      15              0          15
    6      10              4          10
    12      5              8           5    
    18      0             12           0