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

A village has population of 4000 and every person needs $150 \mathrm{~L}$ of water a day.

So, the total requirement of water in a day $=4000 \times 150 \mathrm{~L}=600000 \mathrm{~L}$

Also, it is given that the length of the water tank is $20 \mathrm{~m}$.

Breadth $=15 \mathrm{~m}$

Height $=6 \mathrm{~m}$

Volume of the tank $=$ length $\times$ breadth $\times$ height $=20 \times 15 \times 6=1800 \mathrm{~m}^{3}$

Now, $1 \mathrm{~m}^{3}=1000 \mathrm{~L}$

i.e., $1800 \mathrm{~m}^{3}=1800 \times 1000 \mathrm{~L}=1800000 \mathrm{~L}$

The tank has $1800000 \mathrm{~L}$ of water in it and the whole village need $600000 \mathrm{~L}$ per day.

$\therefore$ The water in the tank will last for $\frac{1800000}{600000}$ days, i.e., 3 days.