Good day everyone. I have a big conundrum here and I urgently need your help:

There is a solution that starts with this:

Let $\displaystyle x $be tens digit and $\displaystyle y$ be the units digit of the original number.

Original number is $\displaystyle 10x+y$ and the sum of its digits is $\displaystyle x+y$.

Number formed by reversing its digits = $\displaystyle 10y+x$.

I can comprehend what it's trying to elucidate on, however, I still do not know

- what is the meaning of the
** units** digit - what is the purpose of having a 10 in the algebraic expressions $\displaystyle 10x +y$ and $\displaystyle 10y+x$

Can you please help me explain? Thank you so much!