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

There is a solution that starts with this:

Let

be tens digit and

be the units digit of the original number.

Original number is

and the sum of its digits is

.

Number formed by reversing its digits =

.

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 and

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