Concept
By a certain percent, we mean that many hundredths.
x percent = x% = x100
25% = 25100 = 14
To express ab as a percent : 
ab x 100 
%
14 = 14 x 100 % = 25%
If A is R% more than B, then B is less than A by 
R(100 + R) x 100 
%
If A is R% less than B, then B is more than A by R(100 − R) x 100 %
Percentage Increase/Decrease
If the price of a commodity increases by R%, then the reduction in consumption so as not to increase the expenditure is :
Reduction % = R(100 + R) x 100 %
If the price of a commodity decreases by R%, then the increase in consumption so as not to decrease the expenditure is :
Increase % = R(100 − R) x 100 %
Results on Population
Let the population of a town be P now and suppose it increases at the rate of R% per annum. Then :
(i) Population after n years : P x 1 + R100 n
(ii) Population n years ago : P 1 + R100 n
Results on Depreciation
Let the present value of a machine be P. Suppose it depreciates at the rate of R% per annum. Then:
(i) Value of machine after n years : P x 1 − R100 n
(ii) Value of machine n years ago : P 1 − R100 n