First, verify that the water consumption of Wenwenjia exceeds 15 tons;

15×2.4=36, that is, the15th ton of water needs 36 yuan. Because 5 1 yuan has been paid, which exceeds 15 tons, and the other part of water is equal to 5 1-36 divided by 3.

That is, (5 1-36)/3=5, that is, the part exceeding 15 tons of water is 5 tons.

So the total water consumption is 15+5=20 tons.

Solution 2

Jia used x tons of water last month.

15×2.4+3(X- 15)= 5 1

The solution is X=20.

So Wenwenjia used 20 tons of water last month.