Xinhua Supermarket: 24 yuan can buy two big bottles and two small bottles and give two small bottles away.

So 1200*2+200*4=3200ml.

Yonghui Supermarket: 10% discount, namely 10*0.9=9 yuan.

2*0.9= 1.8 yuan

Two large bottles 2*9= 18 yuan.

Six dollars can buy three bottles and the rest.

1200*2+300*3=3300ml

Trust-Mart: 30% off for three large bottles of 24 yuan.

1200 * 3 = 3600 ml

So I bought the most in trust-mart stores.