simplification of double summation

Question

I have solved double summation problem,.Kindly check it whether it is correct or not?? $$\sum_{j=1}^3\sum_{i=1}^j (i+j) = 12$$ thanks

Answer 1

$$\sum_{j=1}^3\sum_{i=1}^j (i+j)=\sum_{i=1}^1 (i+1)+\sum_{i=1}^2 (i+2)+\sum_{i=1}^3 (i+3)\\=((1+1))+((1+2)+(2+2))+((1+3)+(2+3)+(3+3)). $$