text: The river speed is 1.5 m / s. What speed relative to the water should the boat have to move
  
  The river speed is 1.5 m / s. What speed relative to the water should the boat have to move perpendicular to the shore at a speed of 2.6 m / s relative to the shore?
  
  Given:
  V rivers = 1.5 m / s;
  V = 2.6 m / s.
  Find: v1.
  Decision:
  Let’s mentally create a triangle of three speeds. The speed of the river is parallel to the bank. A speed equal to 2.6 will, on the contrary, be perpendicular to the coast, and the speed that we need to find will be the hypotenuse between these two vectors, which we can designate the above speeds.
  So we can use the Pythagorean theorem. Let’s make the equation:
  V1 = root of V river ^ 2 + V.
  Next, we substitute the values ​​we know, and we will find the answer.
  V1 = root of 2.6 ^ 2 + 1.5 ^ 2 = 3 m / s.
  Let’s convert to another number system.
  3 m / s = 10.8 km / h