Question

It is a 4-digit number

The tens are double the ones.

The thousands are double the tens.

The sum of the digits is 19.

What number is it?

Answer 1

ones digit --- x
tens digit ---- 2x
thousands --- 4x
hundreds -----y

x+2x + 4x + y = 19
y = 19-7x

but x and y must be single digit positive integers.
let x = 1, then y = 12 , not possible
let x = 2, then y = 5
The number is 8542

let x = 3, then y = 19-21 = negative, no good
any higher number of x will make things even worse

so the number is 8542