Add + 3 levels  
Take away -  3 levels

Number Cruncher
Level 1 add within 10 
Level 2 add/take away within 20

Door Number: Add two numbers.

Sum Dispenser
+ Level 1: TU + U
+ Level 2: Carrying U
+ Level 3: Carrying TU
+ Level 4: Add HTU

- Level 1: TU - U
- Level 2: TU - U
- Level 3: Borrowing T
- Level 4: HTU - TU (Borrowing)