Initial condition :when in air

T cos15 = mg -------(1)

T sin15 = Kq²/r² ------------(2)

from (1) and (2)

tan15 = Kq²/(mgr²) -----------(3)

final condition: when in liquid

let k be dielectric constant..

T'cos15 = mg - B   --------------(4) where B is buoyant force

T' sin15 = Kq²/kr² --------------(5)

from (5) and (4)

tan15 = Kq²/{(mg - B)r²} ----------------(6)

from (3) and (6)

Kq²/(mgr²)  = Kq²/{k(mg - B)r²}

therefore, k = mg/(mg - B)

k = 1.6/(1.6 - 0.8) = 2   :)