Question 608070
Set up a ratio:

( speed 1 ) / ( stopping distance 1 ( = 
( speed 2 ) / ( stopping distance 2 )
----------------------------
{{{ 60 / 200 = 30 / d }}}
{{{ 3 / 10 = 30 / d }}}
{{{ 3d = 300 }}}
{{{ d = 100 }}}
If the car travels at 1/2 the speed it takes 1/2 
the distance to stop, or 100 ft