A certain sum of money is invested at 10%. twice that amount is invested at 8%. the total amount of interest from both investments is $91. how much is invested at 8%?

Let x = Amount invested at 10%

Then 2x = Amount invested at 8%

The formula for simple interest is I = Prt

Where I = interest, P = amount invested, r = interest rate and t = time in years

Since the time is not given so assume it is one year

Rate = 8 / 100 = 8% = 0.08

Rate = 10 / 100 = 10% = 0.10

I = 91

91 = 0.08 (2x) + 0.10 (x)

91 = 0.16x + 0.10x

91 = 0.26

91/0.26 = x

x = 350

Amount invested at 10% is x = $350, and

Amount invested at 8% is 2x = 2x350 = $700