Take any two digit number, say 47
Its first digit is 4
Its second digit is 7
Multiply the first digit 4 by 10
That gives 40
Now add the second digit, 7, to the 40
That gives 47, which is the same 
two-digit number we started with.

Take another two digit number, say 82
Its first digit is 8
Its second digit is 2
Multiply the first digit 8 by 10
That gives 80
Now add the second digit, 2, to the 80
That gives 82, which is the same 
two-digit number we started with.

Conclusion:
Any two digit number is equal to 10 times 
its first digit plus its second digit.

So if the two digit number is written 'ab' 
then 'a' is its first digit and 'b' is its 
second digit. So the two digit number is
equal to 10a+b

Edwin