21 = `1 xx 21`

     = `3 xx 7`

   :. P = {1, 3, 7, 21}

    আবার, 35 = `1 xx 35`

                  = `5 xx 7`

            :.Q = {1, 5, 7, 35}

         :.` PuuQ = {1, 3, 7, 21} uu {1, 5, 7, 35}`

                        = {1, 3, 5, 7, 21, 35}