Velocities must be added by the methods used for the addition of vectors, for example:

A canoeist paddles across a river in a direction perpendicular to the flow with a speed of 1.5 m.s^-1. The current has a velocity 2 m.s^-1 parallel to the river bank. What is the resultant velocity of the canoe?

This problem may be solved by constructing a scale diagram, but since the current and the direction paddled are perpendicular to each other, it is easy to solve it algebraically: