Finding time taken for the following situation.

A ship can travel at a speed of 36 km/h in still water. There is a sea current flowing from east to west at a constant speed of 15 km/h.

Given that the ship is about to travel vertically north, with a distance of 20 km, what is the time taken for the ship to reach its destination?

please include detailed explanations...

you guys, please don't go around saying ...i can do the math, so here's the rough sketch how you do it. PLEASE INCLUDE DETAILS!!!DETAILS! DETAILS!

THANK YOU!!! :)