Add up all the digits
4+5+6=15
The sum of the digits must be equal to 9 so,
15=9
15–9=0
So,it is equal to 6.If we subtract 6 from 456 i.e.,
=456–6
=450
Which is divisible by 9 so,the answer is 6.But if we subtract any digit less than 6 it will not be divisible by 9.
