Danny lowers the sails on his boat. He paddles upstream at 19 km/hr. The current is still running downstream at 15 km/hr. What is the actual velocity of the boat?

4 km/hr

Explanation:

The computation of the actual velocity is shown below:

Because the path of its paddles is opposed to the current direction, the real velocity can be determined by deducting the current velocity to its velocity while paddling

So, the actual velocity is

= Upstream - downstream

= 19 km/hr - 15 km/hr

= 4 km/hr

As we can see it is in positive, so it is an upstream direction  

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