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How many zeros does this end in? (Posted on 2024-04-05) Difficulty: 3 of 5
How many consecutive zeros does:
111000 - 1
end in?

See The Solution Submitted by K Sengupta    
Rating: 5.0000 (1 votes)

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Solution Solution | Comment 1 of 4
My first guess is at least 4 since the binomial expansion will have terms that end in a lot of zeros.  Maybe just 4 but maybe I'm overlooking something and its a lot more.

Working backwards through the binomial expansion of (10+1)^1000 is a bit tedious but gets there:

1^1000 = 1 (the -1 will make this zero)
C(1000,999)*10 = 10000 (4 zeros)
C(1000,998)*10^2=49950000 (4 zeros)
C(1000,997)*10^3=...67000000 (6 zeros)
C(1000,996)*10^4=...24750000 (4 zeros)
all other terms end with have 5 or more zeros, so 11^1000 ends with
...60001

The answer is just 4.

  Posted by Jer on 2024-04-05 11:39:59
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