Answer
Find out the what was the Simons' water bill in September .
To prove
As given
in August the Simons' water bill was $48
in September it was 15% lower
15 % is written in the decimal form

= 0.15
15% of August water bill = 0.15 × 48
= 7.2
September bill = August water bill - 15% of August water bill
= 48 - 7.2
= $40.8
Therefore the Simons' water bill in September be $40.8 .