Any integer divisible by 9 has digits whose sum is divisible by 9. The sum of the digits in 2530 is 2+5+3+0 = 10, and the sum of 1+0 = 1. Having an additional digit "8" in the number, or replacing the "0" with an "8", would make a number divisible by 9. Thus 82,530; 28,530; 25,830; 25,380; 25,308; or 2,538 are all divisible by 9.
99996
No single digit determines divisibility by 3.
The 3, to the left.
4: to make it 90144 = 15024*6
1
29 is not divisible by 3 and how any digit will alter that depends on how that digit is to interact with 29.
99996
It depends on whether you mean the highest digit that you add or subtract or multiply by!
6 (or 0)
No single digit determines divisibility by 3.
The 3, to the left.
There is no digit that can replace the ? in 2?67 to make it divisible by 4. All multiples of 4 are even, 2?67 is an odd number and so cannot be a multiple of 4.
123456
4: to make it 90144 = 15024*6
0.
1
87 is already divisible by 3, so you could add a 0, 3, 6 or 9 anywhere in there and get the same result.