The amount of water that must be added to 45 litres of pure milk at $2\frac{1}{2}$ litres for ₹120 so as to have mixture worth ₹36 a litre is :

15 litres

The correct answer is Option (3) → 15 litres

Initial Milk Quantity = $45 L$

Cost of Pure Milk = $₹48$

Desired cost of final mixture = $₹36/L$

$⇒\frac{45×48+x×0}{45+x}=36$

$⇒45×48=36×45+36x$

$⇒36x=2160-1620$

$⇒36x=540$

$⇒x=15L$