You can  calculate  compound interest  mentally
  Question    Find the compound interest on Rs 2500 @ 12% P.A. for two years 
                   
                     This  method is very useful if time is in  2 units  
     Step   1                First we add  rates    Let it be A
     Step   2                Multiply  rates and divide by 100 Let it be B   
     Step   3                Add A and B  It is our  compound interest rate percentage for two years .Take
                                 percentage
                                

                       12 + 12 +   (  12+ 12  )    =    24 + 1.44 = 25.44
                                                 100

                            Find  25.44 % of 2500 and  that is compound interest

                               625  +  11  =636  ( Again consult  lesson no1 9 for fast percentage  calculations)

                               Rs  636 is our compound interest  

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