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What percentage of radioactive substance is left after five half-lives?

#### Options

20 %

About 97 %

About 3 %

5 %

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#### Solution

**About 3 %**

**Explanation:**

`"N"_0/"N" = 2^5`

`therefore "N"/"N"_0 = 1/2^5`

`therefore "N"/"N"_0 xx 100 = 100/32 = 3.125`%

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