Calculate total volume of displaced water

Since 50 men each displace $4 m^3$ of water, the total volume $V$ raised is $50 \times 4$.

$V = 50 \times 4 = 200 m^3$

Calculate surface area of the tank

The surface area $A$ of the tank is length multiplied by breadth.

$A = 40 m \times 20 m = 800 m^2$

Calculate the rise in water level ($h$)

The volume displaced is equal to the surface area times the height ($h$) of the rise, so $h = \frac{V}{A}$.

$h = \frac{200}{800} = 0.25 m$

Convert height to centimeters

Convert meters to centimeters by multiplying by 100.

$h = 0.25 \times 100 = 25 cm$