Of The local high school had 1,000 bottles of water on hand. The school issued 4
the supply to the 9th grade, ş of the supply to the 10th grade, and 3 of the supply
to the 11th grade. How many bottles remained for the 12th grade?

 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.  

answer: all of the odd numbers were opened

the first person was the one that was opening all of the lockers. since one is odd, this causes all of the odd number lockers to be opened in the end.