Now we need 900 kilograms of liquid medicine with the concentration of 1.5%, and the drug weight is 900* 1.5%= 13.5. Then the weight of 45% liquid medicine is 13.5/45% = 30kg. The total weight of water to be added is 900-30 = 870kg. The final answer is: 30 kilograms of pesticide containing 45% medicine and 870 kilograms of water can be made into 900 kilograms of liquid medicine containing medicine 1.5%.