When building the pyramids, the transport of giant stone blocks was done by exploitation of the good old slave labor. The blocks are transported on an inclined plane with slope α=15°. Whipmaster Jáchym found out that to pull a block up the inclined plane, the full force of 15 slaves is needed. However, when transporting the same block down, he only needs five slaves pulling with full power. Determine the friction coefficient between the block and the inclined plane. Each slave is equally strong and pulls a rope parallel to the inclined plane with constant force.