A Solution of milk and water contains 60% milk.

Given:

Total solution = 40 litres

Milk = $60%$ of 40

$=\frac{60}{100}\times40$

$=24$ litres

Water = $40-24=16$ litres

Let $x$ litres of pure milk be added.

New milk quantity:

$24+x$

New total solution:

$40+x$

According to the question:

$\frac{24+x}{40+x}=\frac{75}{100}$

$\frac{24+x}{40+x}=\frac{3}{4}$

Cross multiply:

$4(24+x)=3(40+x)$

$96+4x=120+3x$

x=24

Correct answer: 24 liters / 24 लीटर