First you divide 12 by 3 getting 4 showing that he took 4 quizzes a week. Now you know the amount of the quizzes he takes a week. Divide the 24 by 4 you get 6 if he takes 4 quizzes a week then it must take his 6 weeks to take all 24. another way is you see that 24 is the double of twelve right? Now double the 3 weeks and and you get six.
So you find the volume (length times width times height) which would be 2,160 then you multiply by 2/3 which will get you to the amount of water they use to fill the tank which is 1,440 cubic inches. I hope this helps! :)