Earth dug out on making a circular tank of radius 7 m is spread all round the tank uniformly to a width of 1 m to form an embankment of height 3.5 m. Then the depth of the tank is

Given, inner radius $\left(r\right)=7\mathrm{m}$

Width$\left(w\right)=R-r=1\mathrm{m}$

Outer radius $R=8\mathrm{m}$

Height of embankment $\left(h\right)=3.5\mathrm{m}$

Volume of cylinder  $=\pi r^{2}h$

$=\frac{22}{7}\times 7\times 7\times h$

Volume of earth dug out to cover embankment $=\pi \left(R+r\right)\left(R-r\right)h$

$=\frac{22}{7}\times (8+7)\times 1\times 3.5$

$=\frac{22}{7}\times 15\times \frac{7}{2}$

$=11\times 15$

$=165$

$154\times h=165$

$h=\frac{165}{154}$

$h=\frac{15}{14}\mathrm{m}$