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Question

The amount of interest earned by an account based on simple interest varies jointly as the principal amount and the number of years the investment has been held. If a principal of $6,000 invested for 7 years earns $3,780.00 in interest, how much would be drawn by $4,000 invested for 12 years?

Answer 1

Let I = Interest, P = Principal, T = Time 

 

Joint variation means I = K*P*T for some constant K.  Use the given information (P = 6000, T = 7, I = 3780) to find the value of K 

 

3780 = K*6000*7  ==>  K = 3780 / 42000 = 0.09 

 

Now use this value of K to solve for I when P = 4000 and T = 12 

 

I = 0.09*4000*12 = $4320