Please try to post the problems in words so that people can search them later and do not post so many problems in one thread. It'll be a mess if different people have doubts in different solutions.
Also most of the GMATPrep problems have been discussed in this forum quite a number of times. Just use the search facility of the forum; It is likely that you will get a solution that suits you.

I'll post the solution of first problem only.
Make separate threads for other problems if you cannot find their solutions.

During an experiment, some was removed from each of 6 water tanks. If the standard deviation of the volumes of water in the tanks at the beginning of the experiment was 10 gallons, what was the standard deviation of the volumes of water in the tanks at the end of the experiment?

1. For each tank, 30 percent of the volume of water that was in the tank at the beginning of the experiment was removed during the experiment.
2. The average(arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons.

Statement 1: If we multiply or divide all the elements by same constant, standard deviation changes by the magnitude of the constant. As it is given that 30% of the volume of water is removed from all the tanks, the standard deviation will also decrease by 30%.

Sufficient

Statement 2: The average (arithmetic mean) volume of water in the tanks at the end of the experiment was 63 gallons does not help in finding the standard deviation of the volumes of water in the tanks at the end of the experiment.

Not sufficient.

The correct answer is A.

mariofelixpasku wrote:shouldnt the standard deviation change exact by that factor if we multiply each volume by the same factor ?

Yes. You are correct.
I've edited my reply.