Two cyclists depart from the same point. One travels due East at 18km/hr while the other travels North-West at 20km/hr. How long (to the nearest minute) will it be before they are 80km apart?

Do you know the cosine rule?

If you have a triangle ABC with opposite angles opposite to side A labelled a, B labelled b and C labelled c, you get:

A^2 = B^2 + C^2 - 2BCcos(a)

Here, A = 80
B = 18t
C = 20t
a = (90 + 45) = 135 degrees

where t is the time you are looking for.

Hi Thank you so much... I didn't think about Northwest as 45 degrees... and that's why was wondering what could the angle. But thanks a lot. I got the answer now.

You're welcome :)