@shuqz,
There is no way as far as i know to set it up algebraically... I set it up exactly how it would look when you added it going down.... For example lets say you got x=100 (not the right answer), now set it up so it looks like this:
100
+
___
7 (visually line the 7 up on the right, my computer didn't line it up the same way i did, sorry) Obviously the last digit of y would be a 7 as 0 +7 = 7 . So now pencil in the 7 and then put another 7 in front of first 7, and just keep going like that... I didn't really understand that part of the question myself, took me awhile... and it took me awhile to come up with x, maybe it could have been done by a spreadsheet or something faster, but i solved it trying every combination possible starting at 283 down... I know I'm not in 250, but I'm happy to get it at all, been awhile since I knew I was right... I also got the 5 digits =11 answer... good luck:)