Take a 4 digit no.(not all digit same)
arrange it's digit in asscending & descending order substract the no.
keep this process on
u always have no 6174
let no is 4000
it's always true
It's actually Kaprekar constant, named after Dattatreya Ramchandra Kaprekar. He was a gifted school teacher. It is said that he came across this number when he was sitting on one of the benches at Dombivli railway station (Dombivli is a city in Maharashtra, near Mumbai)and by looking at the number 6174 on the waggon carrying the goods that was passing by before him.
Now people are doing extensive research in this. They've found Kaprekar routine and also Kaprekar constants in other bases(6174 is the Kaprekar constant in base 10). More about the inventor can be read at: