一件工作，甲单独完成需10小时，乙单独完成需15小时，丙单独完成需20小时，现在甲乙合作2小时后，甲因有事离开了，又过3小时后，丙加入进来，直到工作完成．完成这件工作共用了______小时．
甲乙合作2小时的工作量为2×(
+1 10
)=1 15
；1 3
乙单独做的工作量为3×
=1 15
；1 5
剩余的工作量为1-
-1 3
=1 5
；7 15
则剩余工作所用的时间为
÷(7 15
+1 15
)=4（小时）；1 20
2+3+4=9（小时）；
答：完成这件工作共用了 9小时．
故答案为：9．
| It looks unlikely that medical science will abolish the process of ageing. But it no longer looks impossible. "In the long run," as John Maynard Keynes observed, "we are all dead." True. But can the short run be elongatedin a way that makes the long run longer And if so, how, and at what cost People have dreamt of immorality since ancient times. Now, with the growth of biological knowledge that has marked the past few decades, a few researchers believe it might be within reach. To think about the question, it is important to understand why organisms — people included —age in the first place. People are like machines, they wear out. That much is obvious. However, a machine can always be repaired. A good mechanic with a stock of spare parts can keep it going indefinitely. Eventually, no part of the original may remain, but it still carries on, like Lincoln’s famous axe that had three new handles and two new blades. The question, of course, is whether the machine is worth repairing. It is here that people and nature disagree. Or, to put it slightly differently, two bits of nature disagree with each other. From the individual’s point of view, survival is an imperative. A fear of death is a sensible evolved response and, since ageing is a sure way of dying, it is no surprise that people want to stop it in its tracks. Moreover, even the appearance of ageing can be harmful. It reduces the range of potential sexual partners who find you attractive and thus, again, curbs your reproduction. The paradox is that the individual’s evolved desire not to age is opposed by another evolutionary force, the disposable soma. The soma is all of a body’s cells apart from the sex cells. The soma’s role is to get those sex cells, and thus the organism’s genes, into the next generation. If the soma is a chicken, then it really is just an egg’s way of making another egg. And if evolutionary logic requires the soma to age and die in order for this to happen, so be it. Which is a pity, for evolutionary logic does, indeed, seem to require that. The argument is this. All organisms are going to die of something eventually. That something may be an accident, a fight, a disease or an encounter with a hungry predator. There is thus a premium on reproducing early rather than conserving resources for a future that may never come. The reason why repairs are not perfect is that they are costly and resources invested in them might be used for reproduction instead. Often, therefore, the body’s mechanics prefer lash-ups to complete rebuilds — or simply do not bother with the job at all. And if that is so, the place to start looking for longer life is in the repair shop. |
The word "elongated"(Para. 2) is closest in meaning to ______ .
A．( perpetuated
B．( promoted
C．( stretched
D．( enhanced