How long will it take to ferry 65 people across the river with a big boat and a small boat? The big boat can carry 20 people.

Owner: row across the other side = row to the other side? This is how I understand it. The analysis is as follows:

The big boat can be carried twice, and the remaining 25 people (65-20*2=) can cross the river three times in a small boat (25/10=2.5).

1. A large boat can carry many people, but it takes a long time to go back and forth. Therefore, it is determined to use a large boat to cross the river twice.

Then, it can carry (20*2=) 40 people, and the time is: [(10*2) *2-10=]30 minutes;

2. At the same time that the big ship is loading the first boat, it is planned to use small boats to cross the river three times, so the time taken is: (8*2)*3-8=40 minutes. [Large boat time: (10*2)*2-10=30 minutes. ]

3.

It takes 30 minutes for the big boat to cross the river twice and 40 minutes for the small boat to cross the river three times. Therefore, 40 minutes is chosen as the minimum time. [When the second big boat arrives at the shore (1.5 trips), the small boat will cross the river at the second time. On the way back, when we crossed the river for the third time and reached the shore, we stopped returning.]

Answer: It will take at least 40 minutes to cross the river for 65 people.