Question

The tens digit of a number is twice the ones digit. The sum of the digits in the number is 12. What is the number?

Answer 1

let the number be xy.
$x + y = 12$
$x = 2y \\ x - 2y = 0$
subtract both equations
$3y = 12 \\ y = 4 \\ x + y = 12 \\ x + 4 = 12 \\ x = 12 - 4 \\ x = 8$
so the number is 84.