Count of Multiples of A ,B or C less than or equal to N

Given four integers N, A, B and C. The task is to find the count of integers from the range [1, N] which are divisible by either A, B or C. 

Examples:

Input: A = 2, B = 3, C = 5, N = 10 
Output: 8 
2, 3, 4, 5, 6, 8, 9 and 10 are the only number from the 
range [1, 10] which are divisible by either 2, 3 or 5.

Input: A = 7, B = 3, C = 5, N = 100 
Output: 55 

Approach: An efficient approach is to use the concept of set theory. As we have to find numbers that are divisible by a or b or c. 

• Let n(a): count of numbers divisible by a.
• Let n(b): count of numbers divisible by b.
• Let n(c): count of numbers divisible by c.
• n(a ? b): count of numbers divisible by a and b.
• n(a ? c): count of numbers divisible by a and c.
• n(b ? c): count of numbers divisible by b and c.
• n(a ? b ? c): count of numbers divisible by a and b and c.

According to set theory,

n(a ? b ? c) = n(a) + n(b) + n(c) – n(a ? b) – n(b ? c) – n(a ? c) + n(a ? b ? c)

So. the count of numbers divisible either by A, B or C is (num/A) + (num/B) + (num/C) – (num/lcm(A, B)) – (num/lcm(A, B)) – (num/lcm(A, C)) + – (num/lcm(A, B, C))

Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

// Function to return the
// gcd of a and b
long gcd(long a, long b)
{
    if (a == 0)
        return b;

    return gcd(b % a, a);
}

// Function to return the count of integers
// from the range [1, num] which are
// divisible by either a, b or c
long divTermCount(long a, long b, long c, long num)
{
    // Calculate the number of terms divisible by a, b
    // and c then remove the terms which are divisible
    // by both (a, b) or (b, c) or (c, a) and then
    // add the numbers which are divisible by a, b and c
    return ((num / a) + (num / b) + (num / c)
            - (num / ((a * b) / gcd(a, b)))
            - (num / ((c * b) / gcd(c, b)))
            - (num / ((a * c) / gcd(a, c)))
            + (num / ((a * b * c) / gcd(gcd(a, b), c))));
}

// Driver code
int main()
{
    long a = 7, b = 3, c = 5, n = 100;

    cout << divTermCount(a, b, c, n);

    return 0;
}

Java

// Java implementation of the approach
import java.util.*;

class GFG
{

// Function to return the
// gcd of a and b
static long gcd(long a, long b)
{
    if (a == 0)
        return b;

    return gcd(b % a, a);
}

// Function to return the count of integers
// from the range [1, num] which are
// divisible by either a, b or c
static long divTermCount(long a, long b,
                         long c, long num)
{
    // Calculate the number of terms divisible by a, b
    // and c then remove the terms which are divisible
    // by both (a, b) or (b, c) or (c, a) and then
    // add the numbers which are divisible by a, b and c
    return ((num / a) + (num / b) + (num / c) -
                (num / ((a * b) / gcd(a, b))) -
                (num / ((c * b) / gcd(c, b))) -
                (num / ((a * c) / gcd(a, c))) +
                (num / ((a * b * c) / gcd(gcd(a, b), c))));
}

// Driver code
static public void main (String []arr)
{
    long a = 7, b = 3, c = 5, n = 100;

    System.out.println(divTermCount(a, b, c, n));
}
}

// This code is contributed by 29AjayKumar

Python3

# Python3 implementation of the approach

# Function to return the
# gcd of a and b
def gcd(a, b) :

    if (a == 0) :
        return b;

    return gcd(b % a, a);

def lcm (x, y):
    return (x * y) // gcd (x, y)

# Function to return the count of integers
# from the range [1, num] which are
# divisible by either a, b or c
def divTermCount(a, b, c, num) :

    # Calculate the number of terms divisible by a, b
    # and c then remove the terms which are divisible
    # by both (a, b) or (b, c) or (c, a) and then
    # add the numbers which are divisible by a, b and c
    return (num // a + num // b + num // c -
                 num // lcm(a, b) -
                 num // lcm(c, b) -
                 num // lcm(a, c) +
                 num // (lcm(lcm(a, b), c)))

# Driver code
if __name__ == "__main__" :

    a = 7; b = 3; c = 5; n = 100;

    print(divTermCount(a, b, c, n));

# This code is contributed by AnkitRai01

C#

// C# implementation for above approach
using System;

class GFG
{

// Function to return the
// gcd of a and b
static long gcd(long a, long b)
{
    if (a == 0)
        return b;

    return gcd(b % a, a);
}

// Function to return the count of integers
// from the range [1, num] which are
// divisible by either a, b or c
static long divTermCount(long a, long b,
                         long c, long num)
{
    // Calculate the number of terms divisible by a, b
    // and c then remove the terms which are divisible
    // by both (a, b) or (b, c) or (c, a) and then
    // add the numbers which are divisible by a, b and c
    return ((num / a) + (num / b) + (num / c) -
            (num / ((a * b) / gcd(a, b))) -
            (num / ((c * b) / gcd(c, b))) -
            (num / ((a * c) / gcd(a, c))) +
            (num / ((a * b * c) / gcd(gcd(a, b), c))));
}

// Driver code
static public void Main (String []arr)
{
    long a = 7, b = 3, c = 5, n = 100;

    Console.WriteLine(divTermCount(a, b, c, n));
}
}

// This code is contributed by 29AjayKumar

Javascript

<script>

// Javascript implementation of the approach

// Function to return the
// gcd of a and b
function gcd(a, b)
{
    if (a == 0)
        return b;

    return gcd(b % a, a);
}

// Function to return the count of integers
// from the range [1, num] which are
// divisible by either a, b or c
function divTermCount(a, b, c, num)
{

    // Calculate the number of terms divisible by a, b
    // and c then remove the terms which are divisible
    // by both (a, b) or (b, c) or (c, a) and then
    // add the numbers which are divisible by a, b and c
    return Math.ceil(((num / a) + (num / b) + (num / c) -
                      (num / ((a * b) / gcd(a, b))) -
                      (num / ((c * b) / gcd(c, b))) -
                      (num / ((a * c) / gcd(a, c))) +
                      (num / ((a * b * c) / gcd(gcd(a, b), c)))));
}

// Driver code
n = 13;
var a = 7, b = 3, c = 5, n = 100;

document.write(divTermCount(a, b, c, n));

// This code is contributed by SoumikMondal

</script>

Output:

55

Time Complexity: O(log(min(a, b))), where a and b are the parameters of gcd

Auxiliary Space: O(1)