X bicycled $15\text{km}$ north, then $20\text{km}$ to his left. At least, how many more kms does he need to bicycle to return to his original position?

The minimum distance to return to the original position is the straight-line shortest distance (hypotenuse of a right-angled triangle).
* Vertical distance (North) = $15\text{km}$
* Horizontal distance (Left/West) = $20\text{km}$

Using Pythagoras theorem:

$$\text{Shortest Distance}=\sqrt{15^2+20^2}=\sqrt{225+400}=\sqrt{625}=25\text{km}$$