I'm having a really hard time with this drain problem. Maybe it's the way it's worded, but I don't understand how to set it up.

In case it's hard to read, it says it takes two hours longer to empty a tank using the drain as it takes to fill the tank using a pipe. When it's empty, it's filled with the drain open. If it takes 4 hours to fill the tank under these circumstances, how long will it take to fill when the drain is closed?