Question

A foam plastic of density 580kg / m ^ 3 is to be used as life jacket on river pra.what volume of plastic must be used if it is to keep 20% by volume of an 80kg man above the water in the river. The average density of the man is 1040kg/m ^3.take density of water 1000kg/m^3.

Answer 1

If the life jacket has mass m, then you want to displace 0.80 * (80+m) * 9.81 N of water
now, how much water is that?
I guess you also have to assume that 80% of the plastic is submerged, yeah?