Simple interest

$$I = \frac{PRT}{100} \qquad A = P + I = P\left(1 + \frac{RT}{100}\right)$$ $P$ = principal, $R$ = rate per year (%), $T$ = time in years, $A$ = amount.

Compound interest

$$A = P\left(1 + \frac{R}{100}\right)^n \qquad \text{Compound interest} = A - P$$ $n$ = number of compounding periods. If the interest is compounded $m$ times a year at annual rate $R\%$ for $t$ years: $$A = P\left(1 + \frac{R}{100m}\right)^{mt}$$

Appreciation and depreciation

$$\text{Appreciation: } V = V_0\left(1 + \frac{r}{100}\right)^n \qquad \text{Depreciation: } V = V_0\left(1 - \frac{r}{100}\right)^n$$

Hire purchase

$$\text{Hire purchase price} = \text{deposit} + (\text{instalment} \times \text{number of instalments})$$ $$\text{Extra paid} = \text{hire purchase price} - \text{cash price}$$

Profit and loss

$$\text{Profit} = \text{selling price} - \text{cost price} \qquad \text{Profit \%} = \frac{\text{profit}}{\text{cost price}}\times 100\%$$ $$\text{Discount} = \text{marked price} - \text{selling price} \qquad \text{Discount \%} = \frac{\text{discount}}{\text{marked price}}\times 100\%$$

Tax

$$\text{VAT} = \text{rate} \times \text{price before tax} \qquad \text{Commission} = \text{rate} \times \text{sales}$$ $$\text{Taxable income} = \text{gross income} - \text{allowable deductions} \qquad \text{Net pay} = \text{gross pay} - \text{deductions}$$