Question 1:
Each of these options can be read as "8 to 15", except for 15:8. This is read as "fifteen to eight". Lets say in 8:15 you have 8 of a and 15 of b. In 15:8, however, you have 15 of a and 8 of b. These are not the same ratio.
Question 2:
The amount of people that can go down the slide in 1 minute is 6, because 18/3 = 6. So:
12/6 = 2
6ppl/1min (multiply both terms by 2)
12ppl/2min
It would take 12 people 2 minutes to go down the slide.