NOT SO EASY MATH

#1

Can you please explain the question statement? I am bit confuse how for n=10 output is 9.

#2

Hello, @rahul_bansal,

According to the problem statement, you have a number n and you need to find the count of numbers from 1 to n that are divisible by prime numbers less than 20,

An array of Prime numbers less than 20 are [2, 3, 5, 7, 11, 13, 17, 19]
Now, any number from 1 to n if divisible from any element in the array will increase the count.

Now, let’s take N = 10,

 1 (not divisible by anyone)
 2 (divisible by 2)
 3 (divisible by 3)
 4 (divisible by 2)
 5 (divisible by 5)
 6 (divisible by 2 and 3)
 7 (divisible by 7)
 8 (divisible by 2)
 9 (divisible by 3)
 10 (divisible by 2 and 5)

So, except 1 every element is divisible by some elements from the array.
So, ans is 9

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#3

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#4

CODE: https://ide.codingblocks.com/s/649895

This code is from lecture and giving the wrong output.

PS: Can you please explain this code also.

#5

@rahul_bansal,
okay let me see

#6

@rahul_bansal,

There is one error, below statements need to be written after the second for loop is executed.

if(setBits&1)
ans += n/denom;
else
ans -= n/denom;

Here is the updated code,
Updated Code for Inclusion Exclusion