As stated by the statement, isometric means same measure. So, when a rigid transformation occurs all the measures of the original figure (which will be transformed) will be the same of the new figure (the result of the transformation).
That means that you can verify that the transformation used was rigid by checking the measures of both (original and transformed) figures: if and only if the meausures of the final figure are the same of the original figure the transformation was rigid.
If you choose a blue marble first its the probability of 3/12 simplified to 1/3 and then the probability of choosing a red next is 5/11 as you didnt replace the one blue marble
