Answer to old chestnut: Lisa's dog:

The diagram below shows the answer schematically. A, B, C, D, and L represent the people (L = Lisa), and 1, 2, 3, 4, * represent their dogs, respectively. {} is the boat. Remember that A, B, C, D, L, and * can row.


ABCDL1234* {}
ABCDL34           -> {*12}
ABCDL34                         {} *12
ABCDL34              {*} <-        12
ABCDL34*   {}                      12
ABCDL4            -> {*3}          12
ABCDL4                          {} *123
ABCDL4               {*} <-        123
ABCDL4*    {}                      123
DL4*              -> {ABC}         123
DL4*                            {} ABC123
DL4*                 {C3} <-       AB12
CDL34*     {}                      AB12
CD34              -> {L*}          AB12
CD34                            {} ABL12*
CD34                 {B2} <-       AL1*
BCD234     {}                      AL1*
234               -> {BCD}         AL1*
234                             {} ABCDL1*
234                  {*} <-        ABCDL1
234*       {}                      ABCDL1
4                 -> {*23}         ABCDL1
4                               {} ABCDL123*
4                    {*} <-        ABCDL123
*4         {}                      ABCDL123
                  -> {*4}          ABCDL123
                                {} ABCDL1234*

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