When adding 3 or more numbers, we can add the digits in any order, and the answer will always be the same.

 
Look at these expressions.

3   +   ( 5 + 4 )    
   3    +    9     
is the same as...

( 3 + 5 ) +   4      
8      +   4

** OR **

( 5 + 7 ) +   2      
           12   +   2          
is the same as...

5 +  ( 7 + 2 )   
  5  +   9     

Another Way To Look At It...
 

( 7 + 3 ) + 5  =  7 + ( 3 + 5 )
 10 + 5      =      7 + 8   
15  =  15

** OR **

( 2 + 5 ) + 3  =  2 + ( 5 + 3 )
7 + 3      =      2 + 8
10  =  10

Click on the leaf for practice.