All about flooble | fun stuff | Get a free chatterbox | Free JavaScript | Avatars    
perplexus dot info

Home > Just Math
Count the trailing zeros (Posted on 2021-11-30) Difficulty: 2 of 5
Consider N=82!
How many zeros are there at the end of N?
i. In its decimal form.
ii. In base 9.
iii. In base 11.

No Solution Yet Submitted by Ady TZIDON    
Rating: 4.0000 (1 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Solution solution | Comment 1 of 4
Assuming N is a fixed abstract number, not merely a notation:

In decimal, the number of trailing zeros is the highest power of 5 (which is rarer than a factor of 2) which divides it.

There is one count for every multiple of 5, which is floor(82/5) = 16. Then one extra for every multiple of 25, so add 3.
There are 19 trailing zeros in the decimal case.

In base 9, pairs of 3's in the prime factorization lead to trailing zeros. That number is floor((floor(82/3) + floor(82/9) + floor(82/27))/2), which is 19 also.

In base 11, floor(82/11) = 7 trailing zeros as no factors of 121 are involved.

Alternative interpretation:

The representation 82 itself, before taking the factorial, might be interpreted as a notation in the desired base. If so:

82 in base 9 is 74 in decimal. That would have 17 trailing zeros, the value of floor((floor(74/3) + floor(74/9) + floor(74/27))/2) -- all numbers in the computation being decimal notation.

In base 11, we're looking at what is 90! using decimal notation. It's still not complicated by multiples of 121 being included in the making of the factorial. Floor(90/11) is now 8, as the number of trailing zeros in base 11.

  Posted by Charlie on 2021-11-30 09:08:32
Please log in:
Login:
Password:
Remember me:
Sign up! | Forgot password


Search:
Search body:
Forums (0)
Newest Problems
Random Problem
FAQ | About This Site
Site Statistics
New Comments (0)
Unsolved Problems
Top Rated Problems
This month's top
Most Commented On

Chatterbox:
Copyright © 2002 - 2024 by Animus Pactum Consulting. All rights reserved. Privacy Information