Question

What is the least number that should be added to 864 to makes it exactly divisible by 21?

Answer 1

the answer is 18
you can check
use modular arithmetic or simple division your wish 864 + 18 = 882 which is divisible by 21 as 21 × 42 = 882

Answer 2

$<b><i><u>Answer :</u>$

Given, a number 864

Need to find out the number is divisible by 36

⇒ Prime factors of 36 = 2,2,3,3

So, it is sufficient to find the given number is divisible by 2 and 3.

⇒ Units place in 864 is even number and is divisible by 2 which is also even number.

∴ 864 is divisible by 2

⇒ sum of digits of 864 is 8 + 6 + 4 = 18

⇒ 18 is divisible by 3

∴ 864 is divisible by 3

Hence, 864 is divisible by 2 and 3.