4. The matter is settled. A rower, passing under a bridge, lost his hat. After 15 minutes, he noticed the loss and caught the hat a kilometer from the bridge. What is the speed of the river's current?
4. The rower noticed the loss after 15 minutes, so he will catch up with it in 15 minutes; hence, the hat drifted 1 kilometer in 30 minutes, and the speed of the river current is $2 \kappa$ km/h. (Why will the rower catch up with the hat in 15 minutes? Imagine the situation taking place on a train, with the hat left on a shelf in the carriage... Also note that it doesn't matter whether the rower was rowing downstream or upstream).