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Smallest number with at least n trailing zeroes in factorial
• Difficulty Level : Medium
• Last Updated : 26 Feb, 2021

Given a number n. The task is to find the smallest number whose factorial contains at least n trailing zeroes.
Examples :

```Input : n = 1
Output : 5
1!, 2!, 3!, 4! does not contain trailing zero.
5! = 120, which contains one trailing zero.

Input : n = 6
Output : 25```

In the article for Count trailing zeroes in factorial of a number, we have discussed number of zeroes is equal to number of 5’s in prime factors of x!. We have discussed below formula to count number of 5’s.

```Trailing 0s in x! = Count of 5s in prime factors of x!
= floor(x/5) + floor(x/25) + floor(x/125) + ....```

Let us take few examples to observe pattern

```5!  has 1 trailing zeroes
[All numbers from 6 to 9
have 1 trailing zero]

10! has 2 trailing zeroes
[All numbers from 11 to 14
have 2 trailing zeroes]

15! to 19! have 3 trailing zeroes

20! to 24! have 4 trailing zeroes

25! to 29! have 6 trailing zeroes```

We can notice that, the minimum value whose factorial contain n trailing zeroes is 5*n.
So, to find minimum value whose factorial contains n trailing zeroes, use binary search on range from 0 to 5*n. And, find the smallest number whose factorial contains n trailing zeroes.

## C++

 `// C++ program tofind smallest number whose``// factorial contains at least n trailing``// zeroes.``#include``using` `namespace` `std;` `// Return true if number's factorial contains``// at least n trailing zero else false.``bool` `check(``int` `p, ``int` `n)``{``    ``int` `temp = p, count = 0, f = 5;``    ``while` `(f <= temp)``    ``{``        ``count += temp/f;``        ``f = f*5;``    ``}``    ``return` `(count >= n);``}` `// Return smallest number whose factorial``// contains at least n trailing zeroes``int` `findNum(``int` `n)``{``    ``// If n equal to 1, return 5.``    ``// since 5! = 120.``    ``if` `(n==1)``        ``return` `5;` `    ``// Initalising low and high for binary``    ``// search.``    ``int` `low = 0;``    ``int` `high = 5*n;` `    ``// Binary Search.``    ``while` `(low > 1;` `        ``// Checking if mid's factorial contains``        ``// n trailing zeroes.``        ``if` `(check(mid, n))``            ``high = mid;``        ``else``            ``low = mid+1;``    ``}` `    ``return` `low;``}` `// driver code``int` `main()``{``    ``int` `n = 6;``    ``cout << findNum(n) << endl;``    ``return` `0;``}`

## Java

 `// Java program tofind smallest number whose``// factorial contains at least n trailing``// zeroes.` `class` `GFG``{``    ``// Return true if number's factorial contains``    ``// at least n trailing zero else false.``    ``static` `boolean` `check(``int` `p, ``int` `n)``    ``{``        ``int` `temp = p, count = ``0``, f = ``5``;``        ``while` `(f <= temp)``        ``{``            ``count += temp / f;``            ``f = f * ``5``;``        ``}``        ``return` `(count >= n);``    ``}``    ` `    ``// Return smallest number whose factorial``    ``// contains at least n trailing zeroes``    ``static` `int` `findNum(``int` `n)``    ``{``        ``// If n equal to 1, return 5.``        ``// since 5! = 120.``        ``if` `(n==``1``)``            ``return` `5``;``    ` `        ``// Initalising low and high for binary``        ``// search.``        ``int` `low = ``0``;``        ``int` `high = ``5` `* n;``    ` `        ``// Binary Search.``        ``while` `(low < high)``        ``{``            ``int` `mid = (low + high) >> ``1``;``    ` `            ``// Checking if mid's factorial``            ``// contains n trailing zeroes.``            ``if` `(check(mid, n))``                ``high = mid;``            ``else``                ``low = mid + ``1``;``        ``}``    ` `        ``return` `low;``    ``}``    ` `    ``// Driver code``    ``public` `static` `void` `main (String[] args)``    ``{``        ``int` `n = ``6``;``        ``System.out.println(findNum(n));``    ``}``}` `// This code is contributed by Anant Agarwal.`

## Python3

 `# Python3 program tofind smallest``# number whose``# factorial contains at least``# n trailing zeroes` `# Return true if number's factorial contains``# at least n trailing zero else false.``def` `check(p,n):` `    ``temp ``=` `p``    ``count ``=` `0``    ``f ``=` `5``    ``while` `(f <``=` `temp):``        ``count ``+``=` `temp``/``f``        ``f ``=` `f``*``5` `    ``return` `(count >``=` `n)` `# Return smallest number whose factorial``# contains at least n trailing zeroes``def` `findNum(n):` `    ``# If n equal to 1, return 5.``    ``# since 5! = 120.``    ``if` `(n``=``=``1``):``        ``return` `5`` ` `    ``# Initalizing low and high for binary``    ``# search.``    ``low ``=` `0``    ``high ``=` `5``*``n`` ` `    ``# Binary Search.``    ``while` `(low > ``1`` ` `        ``# Checking if mid's factorial contains``        ``# n trailing zeroes.``        ``if` `(check(mid, n)):``            ``high ``=` `mid``        ``else``:``            ``low ``=` `mid``+``1``    ` ` ` `    ``return` `low`  `# driver code``n ``=` `6``print``(findNum(n))` `# This code is contributed``# by Anant Agarwal.`

## C#

 `// C# program tofind smallest number whose``// factorial contains at least n trailing``// zeroes.``using` `System;` `class` `GFG``{``    ``// Return true if number's factorial contains``    ``// at least n trailing zero else false.``    ``static` `bool` `check(``int` `p, ``int` `n)``    ``{``        ``int` `temp = p, count = 0, f = 5;``        ``while` `(f <= temp)``        ``{``            ``count += temp / f;``            ``f = f * 5;``        ``}``        ``return` `(count >= n);``    ``}``    ` `    ``// Return smallest number whose factorial``    ``// contains at least n trailing zeroes``    ``static` `int` `findNum(``int` `n)``    ``{``        ``// If n equal to 1, return 5.``        ``// since 5! = 120.``        ``if` `(n == 1)``            ``return` `5;``    ` `        ``// Initalising low and high for binary``        ``// search.``        ``int` `low = 0;``        ``int` `high = 5 * n;``    ` `        ``// Binary Search.``        ``while` `(low < high)``        ``{``            ``int` `mid = (low + high) >> 1;``    ` `            ``// Checking if mid's factorial``            ``// contains n trailing zeroes.``            ``if` `(check(mid, n))``                ``high = mid;``            ``else``                ``low = mid + 1;``        ``}``    ` `        ``return` `low;``    ``}``    ` `    ``// Driver code``    ``public` `static` `void` `Main ()``    ``{``        ``int` `n = 6;``        ` `        ``Console.WriteLine(findNum(n));``    ``}``}` `// This code is contributed by vt_m.`

## PHP

 `= ``\$n``);``}` `// Return smallest number``// whose factorial contains``// at least n trailing zeroes``function` `findNum(``\$n``)``{``    ``// If n equal to 1, return 5.``    ``// since 5! = 120.``    ``if` `(``\$n` `== 1)``        ``return` `5;` `    ``// Initalising low and high``    ``// for binary search.``    ``\$low` `= 0;``    ``\$high` `= 5 * ``\$n``;` `    ``// Binary Search.``    ``while` `(``\$low` `< ``\$high``)``    ``{``        ``\$mid` `= (``\$low` `+ ``\$high``) >> 1;` `        ``// Checking if mid's factorial``        ``// contains n trailing zeroes.``        ``if` `(check(``\$mid``, ``\$n``))``            ``\$high` `= ``\$mid``;``        ``else``            ``\$low` `= ``\$mid` `+ 1;``    ``}` `    ``return` `\$low``;``}` `// Driver Code``\$n` `= 6;``echo``(findNum(``\$n``));` `// This code is contributed by Ajit.``?>`

## Javascript

 ``

Output :

`25`

This article is contributed by Anuj Chauhan. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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