How many ways to solve this problem?

[ornate spoke]

Any easy way to solve as i have solution but that's hard to understand for my small brain

[small pike]

I'd represent the initial number as 100x + 10y + 7. The rest should be easy.

Ah, though, a three-digit number won't work.

Well, then I'd try four digits next.

[ornate spoke]

Lemme try your method

[ornate spoke]

Didn't worked

[wary night]

So we know that n ends in 7, right?

We could say that n == 7 (mod 10). Therefore, 5n == 7 * 5 == 35 == 5 (mod 10).

That is, 5n ends in 5.

But then we know that n ends in 57, that is n == 57 (mod 100).

So then 5n == 85 (mod 100), and that tells us n == 857 (mod 1000).

And we can just keep repeating.

Until 5n gives us a number the same length as n that starts in 7.

[ornate spoke]

Thanks I'll have a close look it seems easier

[ornate spoke]

Im trying only but still couldn't reach to answer

[wary night]

Show your work.

[ornate spoke]

Did it got the answer

My dumbass doing subtraction wrong

It was a eay method as compared to bookish one thankss allot for it