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
