Simple interest formula if we start with 1 dollar and double to 2 dollars:

1(1 + i(20)) = 2

1 + 20i = 2

Subtract 1 from each side:

20i = 1

Divide each side by 20

i = 0.05

Now setup the same simple interest equation, but instead of 2, we use 3:

1(1 + 0.05(t)) = 3

1 + 0.05t = 3

Subtract 1 from each side:

0.05t = 2

Divide each side by 0.05

**t = 40 years**