Before the pirates found the treasure checks full of 4,000 doubloons, they had 5,000 doubloons.
What is the percent increase of the pirates' doubloons?
we know that
5,000 doubloons -------> represent 100%
so
Apply proportion
Find out how much percentage represent 4,000 doubloons
5,000/100%=4,000/x
solve for x
x=(4,000*100)/5,000
x=80%
therefore