Now, let's work out problems stated in previous chapters. This will make it easier for you to solve other problems.

We will start with Problem #11 , painting children's wooden furniture. The solution is to paint the tree before cutting it down. A paint solution is poured over the roots of the tree. The solution mixes with the tree's sap and spreads throughout the tree.

Problem #13 , grinding glass sheets, is not difficult to solve. Temporary thin sheets of glass are packed in a thick bundle and ground together.

Problem #16 is about the airplane that made an emergency landing in a field. We should use a dirigible, and we should not use a dirigible. Two big elastic bags are placed under the wings and filled with air. These bags gently lift the airplane. Big platforms on wheels are placed under these bags. Now we can tow the airplane. There is a dirigible, and there is no dirigible the airplane is supported by airbags.

Problem #20 is about the catamaran. It is also not complicated. If you remember, technical systems during the third stage of their development become more dynamic, more flexible, and more organized.

Inventor E. Lapin received Patent #524728 on the catamaran. It had two bodies connected with expandable poles allowing the bodies to be close to each other. This catamaran could also go through shallow parts of the river with greater ease.

Problem #24, about the dredging machine, has a...