# Thread: What is the rule?

1. ## What is the rule?

how do you turn any four digit number to a single digit number using one unchanging rule?

for instance
1000 goes to 3
2000 goes to 3
9000 goes to 4
8000 goes to 5
1234 goes to 0
4321 goes to 0

2. ## Re: What is the rule?

I can't see a direct pattern, but consider this, a number of string length >2 can be reduced to a single number using modular arithmetic.

In your case consider $a \equiv b \mod n$ when $n\in [2,9] \cap \mathbb{Z}$

For example $9 \equiv 4 \mod 5$ as 4 is the remainder of 9 once divided by 5

For longer strings

$39 \equiv 9 \mod 10$

$217 \equiv 1 \mod 6$

$1001200304 \equiv 2 \mod 9$

3. ## Re: What is the rule?

I've seen this type of problem before, so I won't disclose the answer or the pattern. Hint: 7415 goes to 1.

4. ## Re: What is the rule?

Originally Posted by Chook943
how do you turn any four digit number to a single digit number using one unchanging rule?for instance
1000 goes to 3
2000 goes to 3
9000 goes to 4
8000 goes to 5
1234 goes to 0
4321 goes to 0
This is a RIDICULOUS IDIOSYNCRATIC question.
There is no single solution.
I am sure that whoever authored the question knows what he/she means.
But that does not mean that answer is correct.

5. ## Re: What is the rule?

@Plato, the rule's actually pretty simple (it's not like, take the sum of the digits then multiply by 11 and divide by the product of the digits and take the floor value).

I know the solution as well so it's not that obscure. For example:
8244 --> 4
4567 --> 2
8189 --> 5 (Chook943 can verify these to be correct)