Diego deposits $4,200 into a savings account that pays 5% simple interest. he decides not to touch the money until it doubles. how long will diego have to keep the money in this account?

Principal money deposited by Diego into the savings account = $4200
Final amount of money taht Diego will get = (4200 * 2) dollars
                                                                   = 8400 dollars
Percentage of simple interest given = 5%
Let us assume that after x years the money is going to double.
Then
Amount = Principal (1 + rate of Interest * Time)
8400 = 4200[ 1 + (5/100) * x]
8400/4200 = 1 + (x/20)
2 = 1 + (x/20)
2 - 1 = x/20
x/20 = 1
x = 20
So after 20 years the money will get doubled.