You have been asked to walk out onto a newly crusted over lava lake in Hawaii 7.5 days (648 000 s) after the eruption. It is planned to bring in a helicopter with a large drilling rig as soon as the crust is 2 m thick. You carry a small portable drill, which allows you to determine that the crust is 1 m thick. How long will it be before the helicopter and drilling rig can land on the surface of the lava lake?