For the first time, five people sit opposite each other, so one person must row the boat again, plus the rower, one * * * five people, and then row the boat to the opposite side, and then one person can row the boat back, carry four people, and then slide to the opposite side, and so on, and you can cross the river at least six times. Because the boat can't return to the shore by itself, it needs one person to row, so it can only carry four people for the second time. In this way, the boat can cross the river at least six times, each time it takes six minutes, and six times is 36 minutes.

In fact, this interesting problem of crossing the river is a classic problem in mathematics. As early as the Song Dynasty, it appeared in the short story "A tiger gives birth to three children, and there must be a tiger". Later, people adapted various versions according to their own life experiences. Crossing the river is also a common knowledge point in primary schools. One-way and round-trip problems need special attention. People don't need to return for the last time when they cross the river.