A 1000 L tank loses water so that, after t days, the remaining volume is

v(t) = 1000[1-(t/10)]^2 for 0<t<10.

How rapidly is the water being lost when the tank is half full?

I got the derivative fine. -200(1-(t/10))

but then I am having troubling solving.

if the volume is half than v(t) will equal 500, correct? because half of 1000 is 500.

so I solve v(t) = 500 for t?

then input this value of t into the v'(t)?

your help is appreciated!