A counterflow double-pipe heat exchanger is used to heat liquid ammonia from 10 to 30℃ with hot water that enters the exchanger at 60 ℃. The flow rate of the water is 5.0 kg/s and the overall heat-transfer coefficient is 800 W/m2 ·℃. The area of the heat exchanger is 30 m2. Calculate the flow rate of ammonia.