I'm gonna take this question as two different jars being asked for the values of but I very well could be wrong.
So we have 7 white and 3 black marbles. You need to add the values of 7 and 3 to get 10 marbles in total. So next you would set it up as 3/10 which you get .3 and then multiply the .3 by 100 to get 30% chance of drawing a black marble.
Repeat the steps I provided to you in jar #1 for jar #2