Hey there!
When dealing with ratios, you could put your ratio into an actual situation if it helps you envision this a little better; for example, you could say that for 6 people, there's 12 pieces of pizza. That's 2 pieces of pizza for every person.
Using this same example, just figure out which ratio you've been given allows for 2 pieces of pizza per person. Remember that the number of people is the first number and the second number is the slices of pizza.
18:36 allows for 2 slices of pizza per person.
15:9 allows for 1.67 slices of pizza per person.
2:8 allows for 0.25 slice of pizza per person.
3:8 allows for 0.375 slice of pizza per person.
Based on that, your answer is 18:36. Obviously, you don't need to use this example for every ratio problem you do once you get comfortable with them, but it can help you get the grasp of how they work.
Hope this helped you out! :-)
