碘131有八天的半衰期，这意味着每八天其能量就下降一半。
Iodine 131 has a half-life of eight days, meaning that every eight days it loses half its strength.
尽管在过去十年左右的时间里，募集和使用了数以百万美金，野生老虎的数量仍下降一半。
Despite millions raised and spent in the last decade or so, wild tigers may have declined by half over that time.
据摩根大通，2009年第一季度一个月的违约就有15件，但是五、六月份的违约率将下降一半。
According to JPMorgan, there were 15 defaults a month in the first four months of 2009, but the rate fell by half in May and June.
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