Javelle 去了一家咖啡馆，点了一块蛋糕和一杯茶。 不幸的是，收据变脏了，部分内容变得模糊不清。她用餐的收据如下所示。 求茶的价格。 答案 $ ___

1. Find the amount on which the 7% GST was charged.

The GST of $0.65 is $7\%$ of the amount after adding the 10% service charge.

$\text{Amount before GST}=\frac{0.65}{0.07}=9.2857\ldots$

2. Find the subtotal before service charge.

The amount before GST includes the 10% service charge, so it is $110\%$ of the subtotal of cake and tea.

$\text{Subtotal}=\frac{9.2857\ldots}{1.10}=8.4415\ldots$

3. Subtract the cake cost to find the cost of the tea.

$\text{Cost of tea}=8.4415\ldots-5.80=2.6415\ldots$

To the nearest cent,

$\text{Cost of tea}=\$2.64$

However, receipts are rounded to the nearest cent at each monetary row. If the GST shown as $0.65 came from a pre-GST amount of $9.29, then

$\text{Subtotal}=\frac{9.29}{1.10}=8.4454\ldots$

$\text{Cost of tea}=8.4454\ldots-5.80=2.6454\ldots\approx \$2.65$