The sum of the digits of a two digit number is thirteen. If the digits are reversed and the two numbers are added, the sum is 27 less than twice the original number. What is the unit's digit of the original number?

If thetwo digits are reversed and addedwouldn't it still equal 13?

If it is 13, I could just add 27 and divide by 2, couldn't I?

$\displaystyle 13+27=40$

$\displaystyle 40/2=20$

The answer that the book says is 5, but I don't have a clue how it's 5.