To solve problems like this, we must simply break it down and read each part carefully. We can also set it up as a math problem to make it easier to visualize.
The elevator starts on the twentieth floor, so we will represent that with 20.
20
It goes down 11 floors, so we can represent that with a -11.
20-11=9
Now we are on floor 9. Then, we go up 5 floors, so we can represent that with a 5.
9+5=14
Using the math above, we can see the elevator is on the 14th floor.
You first subtract 4 from both sides. Then you divide both sides by -8 (note: when you multiply or divide by a negative the sign flips. Then you get your answer