Pick any 4-digit number at all (should not have the same digit throughout). Rearrange the digits to form as many 4-digit numbers as possible. From the range of numbers (including the one you first picked), subtract the least from the greatest. If the difference is a 2-digit number add the digits together.
The idea behind this is no matter what your answer will always be 9. Yes try it now.
1234 1243 1324 1342 1423 1432
2134 2143 2314 2341 2413 2431
3124 3142 3214 3241 3412 3421
4123 4132 4213 4231 4312 4321
Well, it's 4 digits but I'll go add it... 3+0+8+7 = 18 (1 + 8) = 9.
Worth that 10 minute typing. BTW, the name of the topic reminds me of a book I borrow in the library at my school.
Yeah, I know that it drags to not see you guys again for a long time, and I still hope you remember me. Well, in our place it's summer already, so I will get more active for two months, or even three.
So, I will get those posts counting it's been a long time stuck in the 2000's.