Move the mouse over the text below:

Hover over me A large urn contains [[$100$]] balls, of which [[$36 \%$]] are red and the rest are blue. How many of the blue balls must be removed so that the percentage of red balls in the urn will be $72 \%$? (No red balls are to be removed.)

Note that the position of the tooltip text isn't very good. Go back to the tutorial and continue reading on how to position the tooltip in a desirable way.