Tom K. answered 05/08/21
Knowledgeable and Friendly Math and Statistics Tutor
This is the inclusion-exclusion principle. add ones, subtract twos, add threes, subtract 4s, add 5s, subtract 6s, add 7s, subtract 8s, add 9s, subtract 10s, add 11s. I leave it to you to write all of the elements. For 11 groups, you will have 2^11 - 1 = 2047 groups to show. You might see if you can write the solution without explicitly writing the 2047 groups.