It takes a ship 4 hours to cover 420 km with the current and 6 hours against the current. find the speed of the ship in still water and the speed of the current.

Let s be the speed of the ship, and c be the speed of the current. 

We know that distance equals speed multiplied by the time. 

With the current
     
     or
     
This is our first equation.

Now, against the current, we have
     
     or
     
This is our second equation. 

We solve the equations simultaneously, by adding them together
     

     

Substitute s=87.5 to the first equation to solve for c.
     

     

The speed of the ship is 87.5 km/hr and the speed of the current is 17.5 km/hr.