Answer:
either it's 1:3 or 1:6
Step-by-step explanation:
3 students went to the zoo, and 15 students went to the aquarium.
15 plus 3 is 18.
3/18 then reduces to 1/6
therefore it is:
1:6
(before, I got my answer deleted so, maybe it is 1:3, but I solved it and got 1:6)