The school band is making a large banner for its annual fall concert. After the band members worked for one hour, the band director entered the room and asked how long the banner would be. The trombone section leader replied, "We're not sure. The trombone section and the clarinet section work at different rates, but we are sure that the length of the banner will be 40 feet and the total banner's length." How long will the banner be?

I have read and read this question and still can't understand it. Somebody please help.

Question

The school band is making a large banner for its annual fall concert. After the band members worked for one hour, the band director entered the room and asked how long the banner would be. The trombone section leader replied, "We're not sure. The trombone section and the clarinet section work at different rates, but we are sure that the length of the banner will be 40 feet and the total banner's length." How long will the banner be?

I have read and read this question and still can't understand it. Somebody please help.