Ira S. answered • 08/26/14

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I'm assuming you're compounding continuously and would therefore use the formula A =P*e^rt. So, you know 10000 at 8% for 1 year would bring in A = 10000*e^.08 = 10832.87, so he has made 832.87 in interest. He needs to make 3500-832.87 = 2667.13 on the rest of his money, 30000 or have 32667.13 in his account after the year.

32667.13 = 30000 *e^r divide by 30000

1.088904333 = e^r take Ln of both sides

0.085171991 = r

So the rate on the rest of his money must be approximately 8.52%.