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