You are quite correct. My brainfarts are legendary. When I said 20 and 54, I did of course mean 20 levels and 54 levels...... 20 levels are 1540 balls and 54 levels are 27720 balls. 55 levels are 29260 balls. Simplified, it can be done:

def onelev(x):
'''The x defines number of balls in a single level given the formula f(x)=(x^2)/2+(x/2)'''
return int((float(x)*float(x))/2.0)+(float(x)/2.0))

def tetra(x):
'''The x defines number of levels of balls in the tetrahedron'''
return sum([onelev(y) for y in range(1,x+1)])

tetra(20)
1540
tetra(54)
27720
tetra(55)-tetra(20)
27720

So I guess my answer was in part correct although not very accurate :) But, looking above, there is a smaller answer ( which I haphazardly ignored since I started to work up from 10 :p ) >>> searchtetra(2,18) 560 120 680 14 8 15 My bad there (yeah, don't solve puzzles while working - you don't have time to verify your mess haha :) )

I=I.am()?Code(I):0/0;

A=20, B=54, C=55 or B=54, A=20, C=55 used two recurisve functions and iteration (lazy mans search).

I=I.am()?Code(I):0/0;

BitTorrent ? How quaint... But then he asked about buying, so I guess IF your'e buying, XCode might be the right answer...:cool:

I=I.am()?Code(I):0/0;

Regardless of what the guys are saying about the 24" being too big, I'd say go for it on these grounds: 1) the 24" has a better (full colour spectrum capable) vs. the 20"s reduced colour range display, and is as such better suited for graphics oriented work ("picture perfect"). 2) Although the 20" iMac may seem to be big at first sight, in the long run it becomes small. Too small for serious users. (I've already ordered a Mac PRO and a 30" display...) Buy the iMac with as little memory as you can. Buy it with as much internal disk space as you can afford, since the hard drive is NOT user-replacable (the memory however is). Go to http://www.macsales.com and order: * Memory, 4GB (you want memory for your Mac, and you want memory for windows in Parallels or VMWare Fusion) (there is a original memory return rebate: http://eshop.macsales.com/item/Other World Computing/53IM2DDR4GBK/) * A FireWire Hard Drive, preferrably twice the size of the hard drive inside the iMac. Upgrade the memory. Connect the external hard drive. Time Machine will ask if you'd like to use the new hard drive for you backups. Say yes. Finally, acquire Parallels or VMWare Fusion to be able to run Windows at the same time as Mac OS X, and your´e set. Now you'll have a great iMac, and ready to meet any task your customers may throw at you :) And - your'e all backed up from day 1 !

I=I.am()?Code(I):0/0;