Question

A balloon drifts 20km (60° Sof E) then the engines are turned on and it flies 20km (60° Sof W). What is the total displacement

Answer 1

Drawings always help:

Now we see a triangle with a 120° angle. Use cosine law to find the displacement (orange vector):

$$D=\sqrt{60^2+20^2-60\cdot20\cdot\text{cos}120°}=68\text{ km}.$$