Whoever you are, you should know the answer to an ADDITION problem seriously.
21
15
You need to add 6 - (-9) = 6 + 9 = 15.
12+9=21 21+15=36 36+13=49 49+6=55
15
9 - (-6) = 9 + 6 = 15
30
How about: 9+6 = 15
6
You need to add 12.
first you add 1 by 5 and get 6 then .add 6 by 2 and get 8 .and add 8 by 3 and get 11.add 11 by 4 get 15.add 15 by 5 get 20 its simple just keep adding each number by 1,2,3,4,5 and if the pattern continues keep going
15
They all relate by adding up the 6+6+3=15 You can not get to the sum of 15 is you don't have the first 3 numbers to add up.