A 125 kg flatbottomed boat is floating along at 5.0 m/s, headed east. Sally Sue, standing at one end of the boat, starts to walk at 2.0 m/s, also to the east. If Sally’s mass is 65 kg, what is the speed of the boat as she walks along?

Initial momentum of boat+Sally system is=(125+65)*5= 190*5=950 Kg m/s

Sally's speed is 2.0m/s relative to boat i.e. w.r.t.a stationary observer it is (2+V)m/s where V is the speed of the boat when Sally walks.

Since there is no external force acting on our system, the linear momentum shall be conserved. Hence,
950=125*V+65*(2+V)= 190*V+130
V= (950-130)/190 =820/190=4.3 m/s