len l[] 100
for a = 0 step 6 to 100
   for b = 0 step 9 to 100
      for c = 0 step 20 to 100
         n = a + b + c
         if n >= 1 and n <= 100 : l[n] = 1
      .
   .
.
n = 100
while l[n] <> 0 : n -= 1
print n