Subtract 2-digit numbers: regrouping
1 Subtract 2-digit numbers and 1-digit numbers: no regruping 
2 Subtract 2-digit numbers: no regrouping 
3 Subtract 2-digit numbers and 1-digit numbers
4 Subtract 2-digit numbers through 30
5 Subtract 2-digit numbers through 60
6 Subtract 2-digit numbers through 99
7 Check-up (1) 
8 Subtract three numbers
9 Word problem: subtract 2-digit numbers (1)
10 Word problem: subtract 2-digit numbers (2)
11 Check-up (2)
12 Check-up (3)