In a mixture of 90 liters, the ratio of milk and water is 2: 1. If this ratio is to be 2: 3, then the quantity of water to be further added is:
Quantity of milk = 90 $\times$ 2/3 = 60 liters.
Quantity of water in it = (90 - 60) = 30 liters.
New ratio required = 2: 3.
Let the quantity of water to be added further be x liters.
Then, milk: water = 60/(30+x) = 2 : 3 => 180 = 60+ 2x => x = 60.
$\therefore$ Quantity of water to be further added is 60 liters.