一部手机的标价为 $1288。在以 15% 的折扣售出手机后，店主仍能赚取相当于其成本价 25% 的利润。求该手机的成本价。 答案 $ ... 一台台式电脑的售价为 $2388。
分期付款的价格为：首付 $295，外加 18 个月每月等额付款 $125。计算年单利利率。 答案 ...% 一台笔记本电脑的价值从 2016 年的 $2000 贬值到 2020 年的 $1200。
如果价格每年贬值 $x\%$，求 x 的值。 答案 x = ...

1. Calculate the selling price after a 15% discount on the marked price.

Selling price
$=85\%\times 1288$
$=0.85\times 1288=1094.80$

2. Relate the selling price to the cost price using the 25% profit.

A profit of $25\%$ on cost price means
$\text{Selling price}=125\%\text{ of cost price}.$

3. Find the original cost price by reversing the 25% profit.

Cost price
$=\frac{1094.80}{1.25}=875.84$

1. Calculate the total hire purchase price paid.

Total hire purchase price
$=295+18(125)$
$=295+2250=2545$

2. Find the simple interest charged by comparing hire purchase price with selling price.

Simple interest
$=2545-2388=157$
So, $I=157$ and $P=2388$.

3. Use the simple interest formula to find the annual interest rate.

Time $=18$ months $=\dfrac{18}{12}=1.5$ years.

Using $I=Prt$,
$157=2388\times r\times 1.5$
$r=\frac{157}{2388\times1.5}=0.04383\ldots$
$r=4.38\%$

1. Set up the compound depreciation equation for 4 years.

From 2016 to 2020 is $4$ years.

If the laptop depreciates by $x\%$ each year, the yearly multiplier is $1-\dfrac{x}{100}$.

$2000\left(1-\frac{x}{100}\right)^4=1200$

2. Solve for the yearly depreciation multiplier.

$\left(1-\frac{x}{100}\right)^4=\frac{1200}{2000}=0.6$

$1-\frac{x}{100}=\sqrt[4]{0.6}$

3. Find the percentage depreciation rate.

$1-\frac{x}{100}=0.880111\ldots$

$\frac{x}{100}=1-0.880111\ldots=0.119888\ldots$

$x=11.9888\ldots$

So, to 3 significant figures,

$x=12.0$