Two boats leave port at the same time. One traveling 4 kph at a bearing of 20° and the other at 6 kph ata bearing of 60°. Find how long before the distance between the ships is 10 km.

I've drawn the angles of the boats as they leave the port, the first angle at 20 degrees and the second at 60 degrees. I determined the difference between the two angles is 40 degrees. Now I'm not sure how to find the time it takes for the distance between the two ships to be 10 km, as I only have an angle and a side measure of 10 km so far. Any advice? Thanks in advance.