A metal sphere weighing 60gm in air is suspended by a thread in an oil of specific gravity 0.6. The tension in he thread is 55gmwt, the specific gravity of the  metal sphere is _____________$\times {10}^{-1}$

Given 

The  weight of the sphere in air ${\mathrm{w}}_1=60\mathrm{gm}$

The weight of the sphere in oil ${\mathrm{w}}_3=55\mathrm{gm}$

specific gravity in oil = 0.6 

Relative density at the specific gravity = $\frac{{\mathrm{w}}_1-{\mathrm{w}}_3}{{\mathrm{w}}_1-{\mathrm{w}}_2}$

Substituting the values,

$0.6=\frac{60-55}{60-{\mathrm{w}}_2}\Rightarrow 0.6(60-{\mathrm{w}}_2)=5\Rightarrow {\mathrm{w}}_2=51.6$

Specific gravity in air $=\frac{{\mathrm{w}}_1}{{\mathrm{w}}_1-{\mathrm{w}}_2}$

$=\frac{60}{60-51.6}=7.2=72\times {10}^{-1}$

Hence the correct answer is 72.