$3-155$ A raft is made using a number of logs with

$22$ cm diameter and $2\mathrm{.}2$ m long, as shown

in the figure. It is desired that a maximum

$87$ percent volume of each log will be

submerged when carrying two boys

weighing $440$ N each. Determine the

minimum number of logs that must be used.

The specific gravity of the wood is $0\mathrm{.}78.$