In order to be sure that the results of our experiment can be reproduced, we should

You should repeat the experiment many times to increase precision.

Kinetic energy = 1/2mv^2. since the mass is constant we can form a ratio of it's kinetic energy at 60 divided by that at 30 to see ke(60)/ke(30) = +(60^2)/30^2). this may be said to 300% increase.