We have,
Volume of water require per head per day
 = 150 lit
Total requirement of water in a da = 4000 × 150
= 600000 lit = 600 m³
Volume of tank = (20 × 15 × 6) m³
No. of days in which the water of the tank will last$=\frac{20\times 15\times 6}{600}=3\text{ days }$