Originally Posted by

**c47v3770** "What is the half-life of a radioactive substance that has a decay rate of 25% per decade?

Hint: Remember that the Rule of 70 does not give a good approximation to half-life when the rate is larger than 15%. We must use the actual formula here."

This what I did.

I used the exact half-life formula:

- log102

_____________________

log10(1+r)

In this case, r= 0.25

After substituting, I get:

0.301030 = -2.4094

________

-01250

Am I right? I'm a bit confused..

Thanks guys.

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