某一家庭的每月公用事业费账单是通过根据现行单位单价将电、煤气和水的消耗费用相加计算得出的。下表显示了洪太太（Mrs Ang）家在某一年4月、5月、6月和7月的公用事业使用量。

	每月使用量 (单位)
	4月	5月	6月	7月
电	23	33	49	35
煤气	12	16	22	17
水	16	21	33	26

在4月至6月期间，现行的单位单价保持不变。然而，在7月份，电费单价上涨了7%，煤气费单价降低了5%，水费单价保持不变。已知洪太太在4月、5月和6月的每月公用事业费账单分别为 $103.00、$142.00 和 $209.00，求她7月份的公用事业费账单。

1. Formulate a system of linear equations based on the data provided for April, May, and June.

Let $x$, $y$, and $z$ be the price per unit (in $) of electricity, gas, and water respectively.
We form the following system of linear equations from the table and given monthly bills:

2. Solve the system of equations using a graphing calculator.

Using a graphing calculator to solve the system, we obtain the unit prices:
$x = 2.2$
$y = 2.5$
$z = 1.4$

3. Calculate the revised unit prices for July.

Applying the percentage changes for the month of July, the new rates are:
Electricity: $2.20 \times (1 + 0.07) = \char36{}2.354$
Gas: $2.50 \times (1 - 0.05) = \char36{}2.375$
Water: $\char36{}1.40$ (remains unchanged)

4. Calculate the total utility bill for July and round appropriately.

Total bill for July
$= 35(2.354) + 17(2.375) + 26(1.40)$
$= 82.39 + 40.375 + 36.40$
$= 159.165$
$\approx \char36{}159.17$ (to 2 d.p. as it is a monetary value)