If both A and B are floating, buoyancy and gravity are balanced, and the ratio of buoyancy is 1: 1, which is inconsistent with the meaning of the question.
If both A and B sink, buoyancy F floats = ρ water gv, and the ratio of buoyancy is 2: 1, which is inconsistent with the meaning of the question.
Therefore, it must float for a and sink for b,
FA=mg FB=ρ water gVB..........( 1)
Fa ∶ FB = 8 ∶ 5, that is, FB=5FA/8=5mg/8..........(2).
mg=ρBgVB.........(3)
According to the formula (1)(2)(3), ρB=8ρ water /5 = 1.6 * 10 3 kg/m 3.
So ρ a = ρ b/2 = 0.8 *10.3 kg/m 3.