A radioactive substance has a half-life of 20 minutes. If we begin with a 500 g sample, how much of the original sample remains after two hours?
The easiest way to attack these questions is to start with the original amount of the sample, then draw arrows representing each half-life. Two hours is 120 minutes, so that’s six half-lives. At the end of the stated time period, 7.8 g remains.
500 g ? 250g ? 125g ? 62.5g ? 31.25g ? 15.625g ?7.8125g
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