If it does not exceed 12 ton, his water fee is at most 12*2=24, so it is definitely not the first case. In the second case, let the water consumption be X, the equation12 * 2+(X-12) * 2.5 = 45, and the solution is x=20.5, which exceeds 18, so it is the third case. The equation12 * 2+6 * 2.5+(x-18) * 3 = 45, and the result is x=20. A: The user's water consumption in May was 20 tons.