Question

How many years will it take for $4000 to double at a simple interest rate at 5%

Answer 1

annual interest = .05(4000) = 200
to double, you need another $4000

number of years = 4000/200 = 20

it will take 20 years

to check if you are right you would...
Check:
simple interest on $4000 for 20 years at 5%
= .05(20)(4000) = 4000
the amount = what we had + new amount
= 4000 + 4000 = 8000 which is double what we started with

So the answer would be it took 20 years.