In Nordistan to cross the estuary Bolka, a sea shuttle service to switch from one bank to another through two boats, the Southern Cross and the North Star.
This morning, both boats leave at 6 am sharp on both piers opposite each located on one side of the estuary.
After each passage, they make a 10 minutes stop to disembark and embark passengers and return to their respective home ports.
The two vessels passed them twice during their return. The first cross takes place 100 m from the middle of the estuary and the second crossing 400 m from a side.

What is the estuary width?