looping through possible combinations of McNuggets packs of 6, 9 and 20

[Baba]

level: beginner

exercise: given that packs of McNuggets can only be bought in 6, 9 or
20 packs, write an exhaustive search to find the largest number of
McNuggets that cannot be bought in exact quantity.

exercise source:
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00-introduction-to-computer-science-and-programming-fall-2008/assignments/pset2.pdf

please help me write this code

i believe it's something along the lines of this:

c=0
sol=[]
for n in range (0,10):
 for a in range (0,10):
  for b in range (0,10):
   for c in range (0,10):
    sol=6*a+9*b+20*c
    if sol!=n:
     c+=1
    if c==6:
     print sol