The monthly rent of marilyn's house went from $500 to $400, if p is the percent decrease in the rent, which proportion can be used to calculate p?

For this case we can use the following rule of three:
 $ 500 > 100%
 $ 400 > x
 Clearing x we have:
 x = (400/500) * (100) = 80%
 Therefore, the percentage of decrease in the rent is:
 p = 100-x
 p = 100-80
 p = 80%
 
 a proportion can be used to calculate p is:
 p = 100 - ((400/500) * (100))