Basic Add code:

Load r0 7
Load r1 8
Add r2 r0 r1
Store 9 r2
Halt

Challenge:
add 3 numbers, the one stored in memory location 6, 7, and 8. 
Store answer in location 9

Branching Example:   Add all the numbers between 1 and ???

Put a 1 in location 10 and a 0 in locations 11 and 12.  
Location 10 will be the amount you count by
Location 11 will be the total sum
Location 12 will be the next number to add:

LOAD R3 10
LOAD R0 11
LOAD R1 12
ADD R2 R0 R1
STORE 11 R2
ADD R1 R1 R3
STORE 12 R1
BRANCH 1

Challenge:
See if you can get it to add even numbers starting at 2
See if you can stop the loop after the first 20 numbers
     Hint:  Use  BZERO and put 20 in location 13.  Subtract r3 each time

Solutions:

LOAD R0 10
LOAD R1 11
ADD R2 R0 R1
LOAD R0 12
ADD R1 R0 R2
STORE 9 R1