The load that can be supported by a rectangular beam varies jointly as the width of the beam and the square of itsheight, and inversely as the length of the beam. A beam

12 feetlong, with a width of

4 inches and a height of

6 inches can support a maximum load of

800 pounds. If a similar board has a width of

9 inches and a height of

5 inches, how long must it be to support

1200 pounds?

The board must be

_________

feet