The people of Bridgetown wanted to build a bridge across a nearby river. Since they were poor swimmers, their master Trigonomos agreed to measure the width of the river without actually crossing it. Trigonomos spotted a tree across the river and marked the spot directly across from it. Then he walked to another point 10 meters down the river and found that the angle between his side of the river and the line connecting him to the tree was
40 degrees.
What is the width of the river?