A village having a population of 4000 requires 150 liters of water per head per day.

Solution:

Given that,

Length of the cuboidal tank (l) = 20 m

Breadth of the cuboidal tank (b) = 15 m

Height of the cuboidal tank (h) = 6 m

Capacity of the tank = l * b * h = 20 * 15 * 6

$=1800 \mathrm{~m}^{3}$

= 1800000 litres

Water consumed by the people of village in one day = 4000 *150 litres

= 600000 litres

Let water of this tank last for 'n' days

Therefore, water consumed by all people of village in n days = Capacity of the tank

= n*600000 = 1800000

= n = 1800000/600000 = 3

Thus, the water will last for 3 days in the tank.