How many years will it take for $20 to grow to $100 if the simple interest rate is 4% annually?

Here, Interest Amount = 100 - 20 = 80

Remember, it is Simple Interest (As stated above) not a compound, so

Interest = P * R * T / 100

80 = 20 * 4 * T / 100
80*100 = 80 * T

T = 8000/80
T = 100

In short, Your Answer would be: 100 Years

Hope this helps!