Question 1099154:  Hi 
Ralph opened a savings account and deposit of100 every month after that he deposited 20 why isn't the relationship proportional? 
 Answer by Boreal(15235)      (Show Source): 
You can  put this solution on YOUR website! the amount of money he has is earning interest in different amounts and different proportions each month. 
Suppose he is earning 5% every year compounded monthly. 
After 1 month he has 100(1+(.05/12))=100.42 
he deposits 20 
now he has 120.42, and a month later, that is worth 120.92, fifty cents more, not 42 cents. 
The formula is exponential, but in addition, he is adding a different proportion to the principal each month. 
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