The "subtract and add one" principle is:

The number of whole numbers, inclusive, from whole numbers p through whole
number q where p < q is given by the formula where we subtract and add 1,
getting q-p+1

                  q-p+1
pp.     1-9 -->   9-1+1   =     9 1-digit numbers -->   9*1 =   9 digits
pp.   10-99 -->  99-10+1  =    90 2-digit numbers -->  90*2 = 180 digits
pp. 100-240 --> 240-100+1 =   141 3-digit numbers --> 141*3 = 423 digits
                                                              ----------
                                                              612 digits

Edwin