Cost Price = C.P.
Selling Price = S.P.

 If S.P. is greater than C.P., the seller is said to have a profit or gain.

 If S.P. is less than C.P., the seller is said to have incurred a loss.

 If an article is sold at a gain of say 25%, then S.P. = 125% of C.P.

 If an article is sold at a loss of say, 25% then S.P. = 75% of C.P.

 Loss or gain is always reckoned on C.P.

1. Gain = S.P. - C.P.

2. Loss = C.P. - S.P.

3. Gain Percentage:
  Gain % =Gain x 100/C.P.

4. Loss Percentage:
  Loss % = Loss x 100/C.P.

5. Selling Price: (S.P.)
  S.P. = (100 + Gain %)/100 X C.P

6. Selling Price: (S.P.)
  S.P. = (100 - Loss %)/100 X C.P.

7. Cost Price: (C.P.)
  C.P. = 100/(100 + Gain %) X S.P.

8. Cost Price: (C.P.)
  C.P. = 100/(100 - Loss %) X S.P.

9. 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 %/10)² = (x/10)²

10. If a trader professes to sell his goods at cost price, but uses false weights, then
  Gain % = [Error/(True Value) - (Error) X 100]%