We use mass, volume and density relation as,
[tex]m = V \times \rho[/tex]
Here, m is mass , V is volume and [tex]\rho[/tex] is density.
Also,
[tex]m = \pi r^2 t \times \rho[/tex]
Here, [tex]V = \pi r^2 t[/tex] and t is thickness.
Given [tex]m = 15 kg[/tex], [tex]t = 5 cm = 5 \times 10^{-2} \ m[/tex] and[tex]\rho = 7800 \ kg/m^3[/tex].
Therefore, from above formula
[tex]15 \ kg = 3.14 \times r^2 \times 5 \times 10^{-2} m \times 7800 \ kg/m^3 \\\\ r = \sqrt{\frac{15 kg}{3.14 \times 5 \times 10^{-2} m \times 7800 \ kg/m^3 } } 0.11 m[/tex].
Thus, diameter of the plate,
[tex]= 2 r = 2\times 0.11 m = 0.22 m[/tex]
