Let the water capacity of the tank be x liters. Then,
Initial water consumption rate = x/10 ltr per day
New water consumption rate after the increase by 25% = x/10 * (1 + 25/100) ltr per day = x/10 * 1.25 = x/8 ltr per day
The water tank last in number of days = full tank capacity in ltr / new water consumption rate in ltr per day = x / (x/8) = 8 days.
