The quantity of water that must be added to 36 litres of pure milk at $1\frac{1}{2}$ litres for ₹75 so as to have mixture worth ₹40 a litre is :

9 litres

The correct answer is Option (3) → 9 litres

Cost per litre = $75×\frac{2}{3} = 50$ per litre

Total cost = $36×50=1800$

Let the quantity of the water be x litre.

Total volume = $(36+x)L$

$⇒\frac{1800}{36+x}=40$

$⇒1800=40(36+x)$

$⇒36+x=45$

$⇒x=9$ litres