You may forget.... So let

IMPORTANT FACTS
Cost Price: The price, at which an article is purchased, is called its cost price, abbreviated as C.P. Selling Price: The price, at which an article is sold, is called its selling prices, abbreviated as S.P. Profit or Gain: If S.P. is greater than C.P., the seller is said to have a profit or gain. Loss: If S.P. is less than C.P., the seller is said to have incurred a loss.
IMPORTANT FORMULAE

1.Gain = (S.P.) - (C.P.)
2.Loss = (C.P.) - (S.P.)
3.Loss or gain is always reckoned on C.P.
4.Gain Percentage: (Gain %) Gain % =(Gain x 100) divided by C.P.
5.Loss Percentage: (Loss %) Loss % =(Loss x 100)divided by C.P.
6.Selling Price: (S.P.)
SP =(100 + Gain %)x C.P divided by 100
7.Selling Price: (S.P.) SP =(100 - Loss %)x C.P. Divided by100
8.Cost Price: (C.P.)
C.P. =100 divided by (100 + Gain %)× S.P
9.Cost Price: (C.P.)
C.P. =100divided by (100 - Loss %)multiple by S.P
10.If an article is sold at a gain of say 35%, then S.P. = 135% of C.P
11.If an article is sold at a loss of say, 35% then S.P. = 65% of C.P.
12.When a person sells two similar items, one at a gain of say x%, and the other at a loss of x%, then the seller always incurs a loss given by:
Loss % =(Common Loss and Gain % divided by 10) 2 square =(x/10)2 square
13.If a trader professes to sell his goods at cost price, but uses false weights, then
Gain % ={Error by (True Value) - (Error)× 100} ×%