Given, 3,6,9,12,15,18,21,24,27,⋯
Now, the number of elements in 11th set will be =1+(10)2=21
The total number of elements up to 10th set will be 1+3+...+19=102=100
∴ elements in 11th set =3⋅101,3⋅102,....3⋅121
Sum of these elements =3(101+102+⋯+121)
=3×221×(101+121)=6993