we are given 
A woman walks due east on the deck of a ship at 4 mi/h. 
the ship is moving south at a speed of 24 mi/h
so, firstly, we will draw diagram 
so, we have 


now, we can use Pythagoras theorem

now, we can plug values
and we get 


so, 
the speed of the woman relative to the surface of the water is 24.33 mi/h .......Answer