一位图书作者打算将她的书卖到澳大利亚或巴西，她计划使用新加坡邮政局的特快专递（Speedpost）服务来邮寄她的书。下表显示了邮寄价格和其他要求。

1. Identify the correct mailing costs from the table

An 18 kg parcel falls under the ‘20 kg’ rate.

Australia is in Zone C, so Economy cost = $202.

Brazil is in Zone D, so Economy cost = $282.

2. Calculate the percentage increase compared to Australia

$\text{Percentage increase} = \frac{282-202}{202}\times 100\%$

$= \frac{80}{202}\times 100\%$

$=39.6039\ldots\%$

$\approx 39.6\%$

1. Use the parcel dimension restrictions to find the maximum number of books that can fit

Each book has dimensions $6\text{ mm} \times 128\text{ mm} \times 128\text{ mm}$.

To fit the most books, stack them along the $6\text{ mm}$ thickness.

For $n$ books, the parcel dimensions are
$(6n)\text{ mm} \times 128\text{ mm} \times 128\text{ mm}.$

Largest dimension must not exceed $400\text{ mm}$:
$6n \leq 400$
$n \leq 66.6\ldots$

So the maximum whole number of books is
$n=66.$

2. Check the combined length, width and height condition

For $66$ books, the dimensions are
$396\text{ mm} \times 128\text{ mm} \times 128\text{ mm}.$

Combined length, width and height:
$396+128+128=652\text{ mm}$

Since $652\leq 900$, the parcel satisfies the combined dimension condition.

3. Find the weight of 66 books and identify the correct mailing cost to Australia

Weight of $66$ books:
$66\times 75=4950\text{ g}=4.95\text{ kg}.$

Australia is in Zone C. Since $4.95\text{ kg}$ is up to $5\text{ kg}$, the Speedpost Economy cost is
$\$80.$

4. Calculate the required selling price per book

Mailing cost per book:
$\frac{80}{66}=1.2121\ldots$

Required selling price per book:
$1.80+3.00+\frac{80}{66}=6.0121\ldots$

Since the price is to be given to the nearest ten cents and must be at least enough to earn the intended profit, the lowest suitable selling price is
$\$6.10.$