Purely guessing.

However, there can be more than one answer here. To be precise, 4 different room numbers.

So to get 6 and 1 and also 489 also is guess only. But how to know the answer is correct in the end ?

Yes.You can check through the equation bcda - abcd = 4707 .

6 and 1 is substituting numbers by just trying out.

489 is the result of substituting them into the equation

I see. Noted . Thank @Marhaddict100

@J thank you for your explanation. Let me think about again. Very hard to understand.

Sorry. There are 5 possible answers in total. I'll show you my working. See if you can understand.

100b - 111a + d + 10c = 523


There is actually a logical way to think about this :

Since we need to get 500+ on the right side, this 100b cannot be smaller than the 111a, if not we will get negative value.

So we can make a more educated guess and let b be a bigger digit than a.

What we can do first is to make 100b - 111a become close to 500


Now how much bigger should b be bigger than a?

Ideally, b should 5 larger than a, so that when we do 100b - 111a, we will get about 500.

(Since that would make the difference be close to 5 x 100. 111 is quite close to 100

Eg. If b = 9, a = 4,

9 x 100 - 4 x 100 = 500

Other possible pairings are :
b = 8, a = 3
b = 7, a = 2
b = 6, a = 1



If we let b = 7 and a = 2,

100b - 111a
= 100 x 7 - 111 x 2
= 700 - 222
= 478


Now we have 478 + d + 10c = 523

d + 10c = 523 - 478 = 45



We can distribute this 45 by letting d be and c be 4.

5 + 10 x 4 = 5 + 40 = 45



So another possible answer is : 2745


Check : 7452 - 2745 = 4707

Slowly understand abit of the logical but still need time to study. Thank @J ❤ hoping and hope I can understand all the concept taught by everyone here