Question

I watching this lecture video and I get a wrong result,( skip video to 29:53). link:
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00-introduction-to-computer-science-and-programming-fall-2008/video-lectures/lecture-4
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Does anyone have the source code or can make one the same? I made my own code but it displays the wrong result. can you comment on my source code what I did wrong?
---------------my source code-------------------------------------------
>>> def solve(numLegs, numHeads):
for numChicks in range(0, numHeads + 1):
numPigs = numHeads - numChicks
totLegs = 4*numPigs + 2*numChicks
if totLegs == numLegs:
return [numPigs, numChicks]
return [None, None]


>>> def barnYard():
heads = int(raw_input('Enter number of Heads: '))
legs = int(raw_input('Enter number of Legs: '))
pigs, chickens = solve(legs, heads)
if pigs == None:
print 'There is no solution'
else:
print 'Number of pigs: ', pigs
print 'Number Chickens: ', chickens


>>> barnYard()
Enter number of Heads: 20
Enter number of Legs: 56
There is no solution
------------------It should be-------------------------------------------
Enter number of Heads: 20
Enter number of Legs: 56
Number of pigs: 8
Number Chickens: 12