Question 481433
Amount A=Pe^(rt)

Where P= Principal invested for t years at an annual interest rate r compounded continuously.

Here we given
P = 20,000
A = 40,000
r = 10% (i.e. 0.1)

Substituting given data in above equation, we get
40,000 = 20,000 e^(0.1t)
Dividing both sides by 20,000 we get
2 = e^(0.1t)
ln 2 = 0.1t(ln e)
0.6931 = 0.1t  ( as ln e=1)
so t =  0.6931/0.1
t= 6.931 years (say 7 years)

Ans = 7 years