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

Home > Numbers
A factorial in seconds (Posted on 2017-04-18) Difficulty: 2 of 5
For what value of n will n! be equal
to the number of seconds in 6*n weeks?

See The Solution Submitted by Ady TZIDON    
Rating: 5.0000 (1 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Solution Analytic Solution | Comment 2 of 7 |
6*n weeks = 6*7*24*60*60*n seconds.

There are two factors of 5 in the product, not counting n.  This implies that n is at least 10 since the factorial needs to be at least 10 for two factor of 5.  But n cannot be 10 since that would make the product have three factors of 5.  So n is at least 11.

All the prime factors in the product are at most 7, not counting n.  Then n cannot be 13 or higher, as this would add at least two higher primes to the factorial where the product can only add primes from n, which is only one unless n>=143, which is far too large for the puzzle.  So n is at most 12.

The intersection of the constraints is n=11 or n=12.  Both 11! and 12! have 11 as a factor.  In order for 11 to appear in the product the factor of 11 must come from n.  Then n must be 11.

Pull out the calculator and verify: 11! = 39916800 and 6*7*24*60*60*11 = 3991680.

  Posted by Brian Smith on 2017-04-18 09:49:54
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 (24)
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