SOLUTION: The Knox Valley investment club invested $9000 at 7.8% annual simple interest. How much additional money should they invest at an annual simple interest rate of 5.1%, so that the t

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Question 86252: The Knox Valley investment club invested $9000 at 7.8% annual simple interest. How much additional money should they invest at an annual simple interest rate of 5.1%, so that the total annual interest earned from the two accounts is 5.9% of the total investment?

Answer by scianci(186) (Show Source):
You can put this solution on YOUR website!
Interest = (Principal)(rate)(time), or I = Prt [r as a decimal, not a percent]
We have two accounts involved here ; one with a rate of 7.8% [change it to its decimal equivalent] and a principal of $9000 and another with a rate of 5.1% [change it to its decimal equivalent] and a principal of x. Set up an equation: Prt + Prt = Prt, where the first Prt is for the 7.8% account, the second Prt is for the 5.1% account and the last Prt is for the combined interest, which has to be 5.1% [changed to its decimal equivalent] of the total investment (which is x + 9000). Plug in each parameter and solve for x.