To solve this problem you must apply the proccedure shown below:
1- You have the following information given in the problem above:
- The previous reading was 6,372 ccf.
- The present reading is 6,501 ccf.
2- Then, to calculate the amount of water that Craig's household used during the billing period (which you can call [tex]x[/tex]), you must substract the present reading and the previous reading:
[tex]x=6,501ccf-6,372ccf\\ x=129ccf[/tex]
The answer is: 129 ccf.