The capacity of an elevator is either 20 children or 15 adults. if 9 children are currently in the elevator, how many adults can still get in?

15/20=0.75
if we said that each ( child = 0.75 adult ) 
20 total children num - 9 children in the elevator = 11 children not in the elevator.

11 children * 0.75 adult = 8.25 adults.
that is the math, in real life 8 adults can get in with 9 children.
hope that helps!