Question 810058
Since the current reduces the boat's speed in 
one direction and increases the speed by the 
same amount in the opposite direction,
you can disregard the effect of the current
{{{ d = r*t }}}
{{{ 30 + 30 = r*4 }}}
{{{ 60 = 4r }}}
{{{ r = 15 }}}
The speed of the boat in still water is 15 mi/hr