一根木棒锯成3段需要3分钟，锯成5段需要（　　）分钟．

Skin-diving(潜水) is a new sport today. This sport takes you into a wonderful new world. It is like a visit to the moon. When you are under water, it is easy for you to climb big rocks, because you are no longer heavy.
Here, under water, everything is blue and green. During the day, there is lots of light. When fish swim nearby, you can catch them with your hands.
When you have tank (罐) of air on your back, you can stay in deep of water for a long time. But you must be careful when you dive into deep water.
To catch fish is one of the most interesting parts of this sport. Besides, there are more uses for skin-diving. You can clean ships without taking them out of the water. You can get many things from the deep sea.
Now you see that skin-diving is both useful and interesting.

计算：（﹣3）²+15×（$\frac{1}{3}$﹣$\frac{2}{5}$）+（﹣2）³．