25% of each quantitity is being poured into the other almost three time.
So, let's take each quantity to be 256 litres for ease of calculation.
Initial Stage:
Mixture A ; Mixture B ; Mixture C
Water - 256 litres ; Milk - 256 litres ; Acid - 256. litres
Round 1 -
25% of Mixture A poured to Mixture B.
25% of 256 litres = 64 litres.
Mixture A ; Mixture B ; Mixture C
Water - 192 litres ; Water - 64 litres + Milk 256 litres; Acid - 256 litres.
Round 2 -
25% of Mixture B poured to Acid.
25% of (256+64) litres = 80 litres of Mixture B.
80litres of Mixture B has 25% of 64 litres of water and 25% of 256litres of Milk. (80 litres of Mixture B = 16litres of water + 64litres of milk)
Mixture A ; Mixture B ; Mixture C
Water - 192 litres ; Water - 48 litres + Milk 192 litres; Acid - 256 litres + 16litres of water + 64litres of milk.
Round 3 -
25% of Mixture C poured to Water.
25% of (256+16+64) litres = 84 litres of Mixture C.
84 litres of Mixture C has 25% of 256 litres of acid 25% of 64 litres of milk and 25% of 16 litres of water. (84 litres of Mixture C = 64litres of acid + 16litres of milk + 4litres of water)
Mixture A ; Mixture B ; Mixture C
Water - 192 litres + 4 litres, Milk - 16litrs, Acid - 64litres ; Mixture B - Unnecessary ; Mixture C - Unnecessary
Ratio of Acid in Container/ Mixture A = 64/ 196+16+64 = 64/276 = 16/69