sum = 0
for x in range(50,0,-1):
<tab>print 'iteration %s' % (x)
<tab>num = raw_input('Number please? ')
<tab>sum += int(num)

print 'Your sum is ':str(sum)

notes:
range(50,0,-1) will count down (you said count down)
sum += int(num) is the same as sum = sum + int(num), which accumulates your total sum.
I had to put <tab> in the code since Yahoo strips spaces at the start of lines.

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