A boat crosses the river. The boat has a velocity of 9 m/s. The river has a velocity of 4 m/s. If the river is 75 m wide, how far downriver does the boat land on the opposite shore?

Answer 1

Big assumption, not stated in the problem: The boat is steering straight across the river. If not, start over!

If true, then draw a triangle with the two vectors and their resultant. I'm picturing the river flowing from left to right and the boat trying to move straight up. The vertical side is 9 and the top side is 4. The hypotenuse represents the actual distance travelled in one second. At some later time in a larger but similar triangle, the vertical side is 75m (width of the river) and the top side is the unknown distance on the opposite shore. So by geometry, x/4 = 75/9 and x = 33 m.