Hey there! I’m happy to help!
Let’s first subtract 4 from 20 to see how much was lost.
20-4=16
Now, we see that 16 is a certain percent of twenty that we want to find. When talking about percents, the word “is” represents an equal sign. So, we can write this equation. We will have p represent our prevent.
16=20p (16 is a certain percent of 20)
Now, we solve by dividing both sides of the equation by 20 to isolate the p.
p=0.8
0.8 is 80% or 4/5, so 4/5 of the dam water has been lost.
Have a wonderful day! :D