All you have to do is take $630 and multiply it by 3.9% to get $24.57. Then take $1,047.69 and subtract the initial amount in the account ($630) to get $417.69 then divide that by the interest per month I assume ($24.57) to get 17. So I believe that the account has been held for 17 months. To check simply take $630 multiply it by 3.9% then multiply that by 17. I hope this helped if not please tell me where I went wrong.
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