How long until Heir 2 has twice as much as Heir 1?

[hockeymsg09]

Being the loving person you are you decide to bequeath $8000 each to your only two grandchildren when they turn 16. Grandchild #1 deposits their inheritance into an account that pays 6.85% interest compounded continuously. Grandchild #2 deposits their inheritance into an account that pays 7.6% interest compounded continuously. How long until Grandchild #2 has twice as much money as Grandchild #1?

 

[Denis]

Please do not post same problem twice :shock:

Thank you.