PROFIT AND LOSS
P = Profit/Gain
L = Loss
CP = Cost price
SP = Selling price
MP = Marked price
D = Discount
$\displaystyle P=SP-CP$
$\displaystyle L=CP-SP$
$\displaystyle P\%=\frac{P\times 100}{CP}$
$\displaystyle L\%=\frac{L\times 100}{CP}$
$\displaystyle SP=\frac{100+P\%}{100}\times CP$
$\displaystyle SP=\frac{100-L\%}{100}\times CP$
$\displaystyle CP=\frac{100}{100+P\%}\times SP$
$\displaystyle CP=\frac{100}{100-L\%}\times SP$
$\displaystyle D=MP-SP$
$\displaystyle D\%=\frac{D\times 100}{MP}$
$\displaystyle MP=\frac{100}{100-D\%}\times SP$
$\displaystyle SP=\frac{100-D\%}{100}\times MP$
$\displaystyle CP=\frac{100-D\%}{100+P\%}\times MP$
$\displaystyle MP=\frac{100+P\%}{100-D\%}\times CP$

SIMPLE INTEREST
P = Principal
n = Years
r = Rate of interest
Simple Interest, SI = Same amount of interest every year
$\displaystyle SI=\frac{Pnr}{100}$
Amount, A = Total amount including interest
$\displaystyle A=P+I$
$\displaystyle A=P\left ( 1+\frac{nr}{100} \right )$

COMPOUND INTEREST
P = Principal
n = Years
r = Rate of interest
Amount, A = Total amount including interest
Annually, $\displaystyle A=P\left ( 1+\frac{r}{100} \right )^{n}$
Half-yearly, $\displaystyle A=P\left ( 1+\frac{r}{100\times 2} \right )^{2n}$
Quarterly, $\displaystyle A=P\left ( 1+\frac{r}{100\times 4} \right )^{4n}$
Compound Interest, CI = Interest of one year is added to amount and new interest is calculated
$\displaystyle CI=A-P$

Example
P = 8000
n = 3 years
r = 15% per annum

Simple Interest
Year $\displaystyle I=\frac{Pnr}{100}$ $\displaystyle A=P+I$
1st year $\displaystyle I=\frac{8000\times 1\times 15}{100}=1200$ $\displaystyle A=8000+1200=9200$
2nd year $\displaystyle I=\frac{8000\times 1\times 15}{100}=1200$ $\displaystyle A=9200+1200=10400$
3rd year $\displaystyle I=\frac{8000\times 1\times 15}{100}=1200$ $\displaystyle A=10400+1200=11600$
Total $\displaystyle SI=3600$ $\displaystyle A=11600$

Compound Interest
Year $\displaystyle I=\frac{Pnr}{100}$ $\displaystyle A=P+I$
1st year $\displaystyle I=\frac{8000\times 1\times 15}{100}=1200$ $\displaystyle A=8000+1200=9200$
2nd year $\displaystyle I=\frac{9200\times 1\times 15}{100}=1380$ $\displaystyle A=9200+1380=10580$
3rd year $\displaystyle I=\frac{10580\times 1\times 15}{100}=1587$ $\displaystyle A=10580+1587=12167$
Total $\displaystyle CI=4167$ $\displaystyle A=12167$