The ratio of Milk to water in four types of container of equal quantity is (2 : 3), (3 : 7), (4: 11) and (11 :9) respectively. The four containers are mixed together. What is the ratio of Milk to water after mixing?

For equal quantities, average the milk ratios and water ratios separately. Milk: (2+3+4+11)/4 = 20/4 = 5. Water: (3+7+11+9)/4 = 30/4 = 7.5. Ratio = 5:7.5 = 10:15 = 2:3 = 90:135. Or calculate actual milk: 2/5+3/10+4/15+11/20 = 0.4+0.3+0.267+0.55 = 1.517, water: 3/5+7/10+11/15+9/20 = 0.6+0.7+0.733+0.45 = 2.483. Ratio approximates to 91:149.