Two high-ways cross at 60° angle. On both roads two cars are going at fixed speeds into the same direction, towards a crossing. One at 80 km/h, the other 100 km/h. At the beginning they were 10 km and 8 km away from the crossing. When the distance between the two cars will be the smallest?

My attempt at solving:

And it is not the correct answer, I'm supposed to get a little bit more than five minutes... Obviously the smallest distance between the cars will be when $\displaystyle d^2$ is the smallest..? Also, should I ignore the 30° angle or is it there for something I'll need?

Also the function should be named d(t), not f(t).