there are 122 houses eacg has number on letter box ,how many times does the digit 2 appear on the letter boxes?
single digit : 2 --->1
double digit:
12---->1
20, 21, ... 29,---> 10
32, 42, 52, 62, 72, 82, 92 ---> 7
triple digit
102 , 112, 121, 122 --->4
see if I missed any, then add them up