Math problem in the second grade of primary school: there is a piece of wood, which is 15 meters long. It takes 5 minutes to saw an average length of 3 meters. How long will it take?
First of all, understand the time it takes to saw wood, because it takes 5 minutes to saw it once, and you need to know how many times you saw it. 15 divided by 3 equals 5, which is to find out that this * * * can be divided into five pieces of wood of this length. But it is good to saw it for 5- 1=4 times. The last 5 times 4 equals 20 minutes.