Physics
posted by Randi .
Hey guys  this problem is pretty tough for me...any help? Don't know what to do....
In seawater, a life preserver with a volume of 4.00×10^−2 m^3 will support a 76.0 kg person (average density 980 kg/m^3), with 20.0 % of the person's volume above water when the life preserver is fully submerged. What is the density of the material composing the life preserver? (answer in kg/m^3)
Thanks!

weight displaced salt water= bouyancy force
Volume displace*densityseawater*g=bouyant force
(4E2m^3*density*g+.8*76/980*g)= 76g+
4*E2*density*g
check that and solve for density. Notice the g factor divides out.
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