$m=13 ~tonn=13000~kg$
$d=\frac{m}{V}$ --> $V=\frac{m}{d}=\frac{13000}{720}=18~m^3$
$V=A_b*h$ --> $A_b=\frac{V}{h}=\frac{18}{6}=3~m^2$
$A_b=r^2*\pi$ --> $r^2=\frac{A_b}{\pi}=\frac{3}{3,14}=0,96~m^2$
$r=\sqrt{r^2}=\sqrt{0,96}=0,98 ~m$
$d=2*r=2*0,98=1,96~m$
Volume V = 13.000 kg / 720 kg/m^3 = 18,056 m^3
V = 18,056 = π/4*d^2*h
diametro d = √18,056/(0,78540*6) = 1,9574 m