Question 34690
i think you meant to have: {{{ h=-16t^2+208t }}}?


If so, then when height is 640, we get:
{{{ 640=-16t^2+208t }}}
{{{ 0=-16t^2+208t-640 }}}
{{{ 16t^2 - 208t+640 = 0 }}}
{{{ t^2 - 13t + 40 = 0 }}}
(t-8)(t-5) = 0
so t-8=0 or t-5=0
so t=5 or t=8


Two answers since the arrow reaches 640feet after 5 seconds, going up, then a further 3 seconds (the 8 seconds) it is again at 640 feet, coming down.


jon.