Alright so I got A=4, B=5 and C=9
   A B C          
+ A C B
----------
   C B A
   459
+ 495
--------
   954
        
             
        
        
        
Step-by-step explanation:
Mean of the origibal set = 20
No. of terms = 10
Sum = Mean x No. of terms
 = 20 x 10 = 200
The sum of additional numbers to make the new set of observstion is 4 + 8 + 12...... + 40 = 220
New Sum = 200 + 220 = 420
The no of terms stay the same
New Mean = New Sum/ N
 = 420/10
 = 42