Half of the volume of milk and water mixture of
ratio 7 : 5 is converted into a mixture of ratio 3 : 1
by the substitution (or replacement) method. The
mixture of ratio 7: 5 was formed from the
mixture 7: 3 by adding the water in it. If 240L
milk is required in the replacement method. What
is the total amount of water was added to prepare
the mixtue of 7 : 5?

Let the volume of milk and water in the mixture be '7x' and '5x'
Total volume = 12x
It is given, half of the volume is converted into 3:1
Half = 6x
Milk = 3.5x
water = 2.5x
Later, it is converted into 3:1
milk = 4.5x
water = 0.5x
Quantity of milk is increased by 'x'
It is given that 240L milk is used in replacement process, i.e.
240 - 7(240)/12 = x
x = 100
Amount of milk and water in the mixture is 700L and 500L , respectively.
7:3 is converted to 7:5 by adding water
water added = 500 - 300 = 200L
Hope this helps!