寄信去日本的邮资费率如下表所示。

邮资费率
首 20 g	$0.80
每增加 10 g 或其不足 10 g 的部分	$0.25

陈太太寄了一封重 38 g 的信到日本。她需要支付多少邮资？

1. Calculate the cost for the first 20 g

Cost for first 20 g = $\char36{}0.80$

2. Determine the remaining mass to be charged

Remaining mass = $38 - 20 = 18$ g

3. Calculate the number of additional 10 g blocks

Number of 10 g blocks = $18 \div 10 = 1.8$
Since it is 'per additional 10 g or part thereof', we round up to 2 blocks.

4. Calculate the cost for the additional blocks and the total cost

Cost for additional blocks = $2 \times \char36{}0.25 = \char36{}0.50$
Total cost = $\char36{}0.80 + \char36{}0.50 = \char36{}1.30$