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

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.