Thirty本题问美国有多少成年人面临肥胖问题，根据thirty percent of the adult population has a weight problem．可得出答案。听力关键词：weight problem 或 percent。

In the U. S. A, thirty percent of the adult population has a weight problem. To many people, the cause is obvious--they eat too much. But scientific evidence does not support this idea. Going back to the America of 1910, we find the people were thinner than today. Yet they ate more food. In those days, people worked harder physically, walked more, used machines much less, and did not watch television. Several modem studies have shown that fatter people do not eat more than thinner people. In fact, some investigations report that the fat people eat less than thinner people. Recently, a study of research group found the following interesting fact--thinner people are more active than fatter people. Therefore, those who exercise regularly can eat more and still lose a large amount of body fat.