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Answer: the 12th graders get 150 bottles
how did i get 150?:
we know the total amount is 1,000 bottles of water
since 9th graders got 1/4 of the bottles. 1/4 of 1,000 is 250
10th graders: 1/5 of 1,000 is 200
11th graders: 2/5 of 1,000 is 400
now we ADD all of the sums together: 250+200+400 (doesnt matter which order but do go in order to not confuse yourself) the total should be 850.
FINALLY we subtract 1,000 - 850 = 150 bottles that are left for the 12th graders.