Does water have mass? how do you know?

Yes indeed. Water by golly has mass !

-- I have to do work to raise water up to a higher level from where it is.

-- When water flows over a paddle wheel, it can move a boat or do other work.

-- When I let water pour down through a turbine and turn a generator, I get electrical energy out of the generator.

-- When I put a bucket of water on one end of a see-saw, I have to put some mass on the other end to make it level.

None of these things could happen if water had no mass but energy is conserved.

And finally the kicker:

-- When I put a bucket or a water balloon on the scale, the scale says there are some pounds of weight on it.

I know that Weight=(mass)x(gravity).

So if the scale says there is weight, then there must be mass on it !

carla slowed down in the beginning of her trip, stopped, and then sped up

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s